The spear, pike, halberd, when not used in a phalanx are typically best suited against cavalry/horse based combatants. The typical Roman foot soldier (legionnaire) would be using either gladius or spatha, in conjunction with a scutum, which was a devestating combination that not only enabled great versatility in melee combat, but also against ranged (especially when implementing formations like the Testudo).

Now, the Greeks, as you mentioned, used a shield more similar to the Parma (I don't know if it has a unique name, if it does, I can't recall it), which while wide, did not offer the mobility and formative nature of the Scutum. And while they did use weapons like the Xiphos, it was secondary to the Dory, a long, thrusting spear. Great if your opponent is further away, but quite the liability in close quarters.

The point I'm trying to make is that it may be less about the sword vs. spear debate in the Romans vs. Greeks that you put forward, and more, shield vs. shield.

Spears would long have a place in warfare, but you cannot have an army made up exclusively of them, same goes for sword and ranged.

Some sources for historical warfare and weaponry.

A good little website for quick references, I've found their stuff to be accurate and best of all, free http://www.ancientmilitary.com/roman-weapons.htm

http://www.ancientmilitary.com/spartan-weapons.htm

Required reading:

Peter Connolly - Greece and Rome at War (1981)

https://openlibrary.org/books/OL693053M/Greece_and_Rome_at_war

Most of my knowledge on Roman and Greek warfare and weaponry has come from this book. I've attached an Amazon link for the book, and you may want to hop(lite) on it as I'm not sure if it is still in print.

Peter Connolly is a great author, and has produced numerous books, across a variety of topics, but his main focus was Greco-Roman history.

http://en.wikipedia.org/wiki/Peter_Connolly

The Cambridge History of Greek and Roman Warfare http://www.cambridge.org/ca/academic/subjects/classical-studies/ancient-history/cambridge-history-greek-and-roman-warfare-volume-1

Now, the above book is expensive. Like college textbook expensive, but it really is comprehensive and is directly related to the topic at hand. I can't think of a better resource out there. Obvious Cassius Dio has a wealth of knowledge in there, but it covers far more than combat, and is more useful as a source on the politics.

I tried to google a cheaper alternative and found http://www.amazon.com/Warfare-Classical-World-Encyclopedia-Civilizations/dp/0806127945/ref=cm_lmf_tit_1

I haven't read it myself, but it has received positive response, and it sounds like there is a heavily illustrated one out there as well.

EDIT: Forgot about the Amazon rule, apologies. Removed the link and changed it to an openlibrary link. To those interested though, Greece and Rome at War is available on Amazon.com and is worth picking up.