Personally, against the Mongols, I'd much rather have Longbowmen. I tend to fight defensively against them, and the stakes are really useful.

Really, Longbowmen and Pavise Crossbowmen are both easily powerful enough to conquer Europe. In the hands of a skilled player, both units can be incredibly deadly. On the other hand, against the computer, you could conquer Europe without any missile units at all, if you were willing.