Critics of Milton have tended to put him up against Shakespeare. Indeed, Milton imagined himself under the influence of Shakespeare, who he deemed to be a poet of great stature. In L’Allegro, Milton writes that poetic inspiration comes:

If Jonsons learned Sock [hat] be on,
Or sweetest Shakespear fancies [fancy’s] childe,
Warble his native Wood-notes wilde,

He is even more adoring in his sonnet, “On Shakespeare”, which was included in the Second Folio:

What needs my Shakespeare for his honoured bones,
The labor of an age in pilèd stones,
Or that his hallowed relics should be hid
Under a star-ypointing pyramid?
Dear son of Memory, great heir of fame,
What need’st thou such weak witness of thy name?
Thou in our wonder and astonishment
Hast built thyself a live-long monument.
For whilst to th’ shame of slow-endeavouring art,
Thy easy numbers flow, and that each heart
Hath from the leaves of thy unvalued book
Those Delphic lines with deep impression took,
Then thou, our fancy of itself bereaving,
Dost make us marble with too much conceiving; And so sepúlchred in such pomp dost lie,
That kings for such a tomb would wish to die.

But Milton has not always been seen in the image of Shakespeare’s greatness. In his seminal study on Milton’s poetic influence, T. S. Eliot fired the starting gun for a twentieth-century rebellion against Milton’s place in the canon. T. S. Eliot’s primary charge against the ease of reading Milton – later to be taken up by F. R. Leavis in Revaluation – is that Milton’s syntax is convoluted “through the hypertrophy of the auditory imagination… it would seem necessary to read it [Paradise Lost] in two different ways, first solely for the sound, and second for the sense.” Milton dictates less interesting imagery than Shakespeare, but fits it more laboriously into a metrical scheme. So enslaved to euphony, Milton produces a distinctive style. It

is not a classic style, in that it is not the elevation of a common style, by the final touch of genius, to greatness. It is, from the foundation, and in every particular, a personal style, not based upon common speech, or common prose, or direct communication of meaning… In Milton there is always the maximal, never the minimal, alteration of ordinary language. Every distortion of construction, the foreign idiom, the use of a word in a foreign way or with the meaning of the foreign word from which it is derived rather than the accepted meaning in English, every idiosyncrasy is a particular act of violence which Milton has been the first to commit… As a poet, Milton seems to me probably the greatest of all eccentrics. His work illustrates no general principles of good writing; the only principles of writing that it illustrates are such as are valid only for Milton himself to observe.

Thus he argues that Milton is difficult to read because of his difficult syntax, his foreign borrowings, and his attachment to etymological meaning. I don’t want to stray too far into the critical history surrounding Milton, but there is a lot of truth in what Eliot had to say; his criticisms of Milton’s sentence structures are simply accepted by many readers today. Whatever the effects of Milton’s "Grand Style", Helen Gardner says, it “is highly artificial”. “To put it crudely, it is impossible to read Paradise Lost aloud without going far beyond the range of one’s normal mode of speech.”

Certain features of Milton's sentence structure, such as inversions, are indeed beyond the the range of normal speech. Yet so were many of Shakespeare's lines. His sentence structures correspond to a liberty that early modern writers were vouchsafed as part of the language they were writing in. Insofar as Milton had a preference towards a certain idiom, these critics are right to point out that it was his own, but this makes it is difficult to account for in a ‘historical’ way.

I think at least part of such an answer may be contained in his early education. He was borne to an educated father, privately tutored, studied at St. Paul's and Cambridge, and was a voracious bilbiophagist until the compulsion rendered him blind. The curriculum at St. Paul's was a steep and demanding classical education. It is probable that he entered at the age of 7 or 8, but there is still no definitive answer on this point. His stay at St. Paul’s was extended for reasons likewise unknown.

His education there was both wide-ranging in the texts he had to read and required something of a real understanding of the ancient world. There was a burden on the student not just to know about the ancient world, but to share certain of its prejudices and values. The best (and perhaps only) extended study of Milton’s time at St. Paul’s, John Milton and St. Paul’s School, by Donald Lemen Clarke, has some choice quotes to furnish the point. There was

a preoccupation with language, with purity of vocabulary, correctness of grammar, and beauty of style. If taken alone they might well lead the modern reader into the misapprehension that the humanist grammar school was concerned only with language and quite ignored content. To the contrary. But the humanists did believe that the boy in school should master the arts of communication before he concentrated on the matter to be communicated.

And:

in preparation for them [his early surviving writings] he [Milton] must have spent many hours at the imitative exercises of memorizing, translation, and paraphrase which the school curriculum planned as first steps toward the original composition in Latin and English which graduates of the school could look forward to.

Now this schooling may have been somewhat unexceptional when compared to the schema for other institutions at the time – and indeed, Milton’s education would have been in harmony with the theories and pedagogic strictures of his day. We might hypothesise that the education of Shakespeare and Milton did not differ greatly (there is no record of Shakespeare’s education). Yet even if we were to assume – fairly – that Shakespeare attended the grammar school in Stratford, Milton may have distinguished himself by his work ethic. One of his recent biographers, Anna Beer, is on-the-money as far as a straight historical answer goes when she says:

It is easy to feel overawed by the amount of material that the young John Milton absorbed as a child and to pass over his vast reading with the simple statement that he was a prodigy. A more accurate assessment is that he was merely working, admittedly extremely hard and with great success, within the conventional educational models of his time.

Milton's productions seem to have owed much more to his humanist education than Shakespeare’s writing owed to his. The ecumenical and ranging character of Shakespeare’s sources and imagination mark out a certain contrast with Milton's early reading (though he did not record his bookish in-take at this early age). Laurence Olivier famously remarks that Shakespeare was “the nearest thing in incarnation to the eye of God”; we might conjecture that Milton, by contrast, retained and put more stock on the lessons of literary economy and clear argumentation that he had dedicated such demanding study to as a boy. These principles would have been important to a man who, by all accounts, was earnest in his desire to “justifie the ways of God to men”: PL is a polemic work.

Indeed, in its protean form, PL was a typical drama. Perhaps the greatest indication of Milton’s indebtedness to the classical world is that PL eventually takes a form modelled on the Aeneid and, in so doing, achieves its status as one of the few truly remarkable epic poems in the English language. PL's abnormally long sentences and Milton's preference for negative constructions also suggest Latinism.

A learned sense of economy would also explain T. S. Eliot's most explicit unfavourable comparison between Milton and Shakespeare. Whereas Shakespeare’s “combinations of words offer perpetual novelty… [and] enlarge the meaning of the individual words joined”, Milton writes loose and generic images. Eliot famously tells us that “Milton writes English like a dead language”. There is a degree of truth to the idea that Milton’s combinations of words are less challenging than Shakespeare’s, but statistical analysis shows a surprising number of adjectival modifiers in both his poetry and prose. On this point, the similarity between the two writers may be greater than Eliot thought.

Milton compensated for his concision by choosing nouns loaded with etymological meaning. This diminished his need for compounding and chimes with what Eliot was to argue later: Milton's poetry is eminently Latinate. This propensity towards economical language – formed in part by his educational heritage – is not, however, the whole of the story. I can think of many instances in Paradise Lost where, attempting productive combinations, Milton gives the modern reader difficulties.

Finally, I know there is little distinction between the two in the abundance of that quintessential Early Modern word - 'Thou'. Both writers use the informal second person singular pronoun despite its very sudden fall from favour. In Paradise Lost the word comes with almost every major speech; in "On Shakespeare" it occurs 3 times in 14 lines. Milton’s employment of it also means that there is none of the concomitant reduction in the -st and -est preterite suffixes. As anyone who has ever dropped into a mock-Shakespearian demotic knows, these preterite suffixes are crucial to the effect and I side with /u/Nougattabekidding in suggesting that the linguistic differences might easily be overstated.

I'll have a think about this more today and see if I can add anything later, but I hope that this can kick things off.