This question has been asked so many times.. Each user will tell you different. The best thing to do is try each one out. One you will like, the other you will prfer more. Basically its all up to the user. Each product has its advantages and disadvantes in each area. Either way, Bio}{azard (coder of XBConnect) and TheDaddy (Coder of Xlink) deserve many props in all they have done for the tunneling world.
While it is with no doubt, the main games you will find on XBConnect is halo2. We have always been strong in Halo series. While thats good if you like playing halo, if you prefer other games, Xlink seems to be more on that. I still love the occassional Rainbow or ghost recon, its just tougher on XBC to gather a crowd.
Lag, well no matter what anyone says, its all based on Location, people in the room, and the host. Both applications use the same packet drivers to deliver the packets.
Try them both, either way you cant go wrong in helping out the Xbox tunnels.