Trying to estimate this kind of comparisson is actually quite difficult to do with the sources we have available to us, largely down to the ways in which England's military strength was organised and levied, and the comparative strategic focuses of, say, Æthelstanian England with its contemporary Ottonian Germany. Any kind of "who would win" scenario would be predicated more on context than it could be determined in terms of raw military power.

The German 'military' of the 920s and 930s, so far as you can conceptualise it as a single unified force, was itself undergoing somewhat of a reorganisation under Henry the Fowler and, more substantially, Otto I in response to decades of Magyar raiding, recorded in the Chronicons of both Regino of Prüm and Thietmar of Merseburg. The Ottonian army as mentioned in Thietmar's Chronicon is one transitioning increasingly towards heavy cavalry and the use of 'shock' cavalry charges, largely itself a response to the light cavalry strategy commonly employed by Magyar raiders. In stark comparison, the contemporary English army is heavily based disciplined infanty formations, and indeed would be until 1066, or even well into the 12th Century, although there does seem to have been a West Saxon mounted contingent at the Battle of Brunanburh. This doesn't necessarily imply that it was inferior: the Danish force which attacks Rochester in 885 had previously campaigned in France where they had defeated a French cavalry force and captured sufficient horses to 'become a mounted army', and were subsequently defeated by Alfred's mostly-infantry force. Indeed, a good example of the strength of an English infantry force can be seen at the Battle of Hastings in 1066, where the English shieldwall withstands multiple Norman cavalry attacks.

The real problem in comparison, however, comes from the fundamental nature of the English armies of the 10th century: while the English army did wage offensive warfare, the bulk of its strength was inherently defensive. The Alfredian reforms of the 870s which created the system of Burghal fortresses garrisoned by fyrd militias, and military fleets crewed by lithmen, had substantially transformed the ability of the English state to exercise political and military control and raise military forces, but the scope of these forces were largely designed to be *re-*active. While English armies retained their professional core of elite soldiery, the force projection of which 10th Century England was capable revolved around the ability of its armies to respond rapidly in force to apparent threats, and this in turn was dependent on the fyrd. A localisation of the gesith networks of personal loyalty from which the King's personal warband and army were drawn, the fyrd was tied intimately to local communities and organised and distinguishable at levels as small as the Tything or town. While these forces coalesced at shire level and often combined with forces from other shires in order to respond to larger threats, it would have been extremely unlikely for them to serve in an offensive role.

What could be said, therefore, is that an Ottonian force which, for some reason, decided to attack England, would be faced with a powerful naval defence and then a land army which would have been able to quickly rally a disciplined jnfantry force against it which may even have previously fought a mounted enemy, while an English force which attacked Ottonian Germany would likely find itself quickly outmanouvred and outnumbered.