Old Ben is better, for sure. Didn't used to be that way, but ever since the rework gave Ben a taunt, St Han's usefulness as a new player's rebel tank has fallen. These days it might be best to just build St Han up enough to get through the CLS event (G8-9) and then ignore him after that.
Might be worth considering running both though, since on defense Old Ben does mind tricks first, then taunts, so you have a period at the start with no taunt if you just use him. Who I'd try removing depends on what you're facing - if it's teams that punish AoEs (Rex lead, CLS lead, zMaul lead, etc), then Lando; if not, then Leia. Either way though, might not work out for the better, in which case switch back.