I've used both programs for years, and I like each one for different reasons.
Xlink Kai is cool because I can set it to the Americas section of halo 2 system link and choose between +30 games from my tv screen. The downside of this software is that there is no easy way of finding a low ping host.
XBC is cool because I can easily find a low ping server, but I don't like that I have to go over to the computer, ask my sister to let me fix something real quick, find another server, and then finally get back in the game.
If Xlink allowed ping filtering, then I'd say Xlink hands down.