Altair doesn't match either one in athleticism (in or out of combat). The most he's ever done to compare is run a step and a half up a wall, the likes of which either Prince bound between, run across, and launch themselves off of casually. SoT Prince doesn't even need the sands, let alone does PoP Prince require Elika. The only advantage in this race Altair actually has (if it can be called that) is the fact that he's been to Jerusalem before.
hm this is a hard one, but i give it to prince, because he can run accross walls and altair never did that, assasins cread was more of a realistic game all the things altair did was possible, and the stuff prince does is not really "imposible" but exagerated to an extent that it makes it imposible. so prince wins
Edit: ok not everything that altair was posible, like falling as far as he does and landing in some hay would killyou, unless it was a lot of hay and it wasnt all that much, but other than that and the whole mystical fight at the end all is possible