That is a tough one!
Wow--tough question. They really are both excellent. It took me a while to like Seasun, so I'd say you should go with Kristin first. She is more like Cathe in athleticism, whereas Seasun is a bit more choreographed. I think Kristin is a bit better at cuing, too, although that may just be because her choreography isn't quite as dancy, so its sort of easier moves to cue in the first place.
I actually like Step This Way 2 a tiny bit better than the original, just because it is hardly broken down at all. If you pick up complex choreography easily, I would highly recommend that DVD or tape!
Whatever you decide, I think you will be happy.