It really depends on how big you (or your buddies are that will be riding). I currently have a super old jetski, I think it's a 650cc and that can get a rider that weighs probably 215-225lb max (with a longggg run through to get on plane).
All of the new skis will be more than enough, 130hp and up is plenty to get any sized skater out of the water. The GTI is a good option, but in all seriousness that's the benefit of skating behind a ski, you really can't go wrong with anything - it almost comes down to what brand is the most reliable/durable. But all in all, if you can swing it, find a 4-stroke engine - they seem to last longer.
(Message edited by sinkoumn on March 21, 2008)