My vote goes for the Medieval army for a few main reasons. First off, Roman armour is vastly inferior when compared to Medieval armour. As such, Roman weaponry would have a much more difficult time piercing the armour of their opponents while the opposite would be true for the Medieval army. Metal working went through many revolutions between the time of the Romans and the rise of the knight in Europe. The second major issue, as tess brought up, is the issue of mobility. The Romans were primarily foot-borne infantry using short swords versus a mix of cavalry and infantry using lances, long swords and pikes/halberds. In other words, not only would the Romans get knocked around by the foot-borne soldiers, they'd also have cavalry riding all around them hacking them to pieces. In short, my money goes with the Medieval army.
