We have two Responses in the family. My dad has a '06 LXi, I have a '05 LX, and our neighbor has '03 LXi. One of our close friends also has a '98 LX. I've honestly done some of my best skiing behind the 98. There are a variety of skiers that all ski our boats from 15' off to 32' off and they all perform well.
I do notice a slight difference with my dads '06 LXi though. The wake is just slightly softer. Softer is the term I'd stick with. Less sharp maybe. I still think that the '98 LX has the smallest wake out of all of them. Maybe it's because there is no rear ski locker and the engine sits different that the 2000 models and beyond.
I was looking to upgrade in the next few years but from the sounds of it you should probably stick with either an '03 to '06 or jump to 2012 and newer. Good luck!!