Han overall works better as a tank, compared to Roshi, who fits in a more offensive role. Han does however have very valuable use due to being able to remove all buffs and shields (of both teams I think?) in round 1, while Roshi is able to remove barriers but not buffs/shields.

As a whole, I would recommend Roshi if you can't decide, since he can fit into a lot of different teams, and can be used far into the game even when stronger units are eventually added.