Xander or Ryoma?

It depends. Ryoma is an infantry unit with high Atk/Spd while Xander is a cavalry unit with high Atk/Def. In my opinion it really doesn't matter which one you go with as long as it fills the role you need.

(I'd personally choose Ryoma though....lobster god :D)

Depends on your team composition. If you're going for a hone cavalry team Xander is a better choice, otherwise I'd stick with Ryoma.