The only game I play on tunnels is Halo, so this is purely from my Halo point of view

Kai is smoother for the most part, though I've noticed a few people have better hosts on XBC than XLink. But overall, XLink seems smoother.
XBC has more users than Kai, though Kai's userbase will grow a lot once it becomes mandatory and the masses that are still using RC2 arrive. So no real winner there.
XBC has more features. Do you need them? Personal preferance, you decide

If you like XBC and you like XLink too, like I do... Then do what I do, keep them both and use them both. If I just decide to play a game and have nothing arranged with anyone, then I open both apps and look which will get me a game fastest (so if there's rooms with space for me on XBC, or if there's UK PAL people in the room on Kai) and go with that one...
It's all personal preferance, but posting that here will get a 99% Kai vote, so maybe it's best to try for yourself after you've read everyones opinions.
