I can't speak for the 98' GTS, but it looks like it would be slow with the 720.
96' - 97' GTX is a great well rounded ski, comfortable with a passenger and can reach mid to lower 50's. Big 15 gallon tank, can handle the chop very well, great for cruising distances, plenty of storage etc. Carb 787 is easy to work on and won't break the bank. It's not as tight like a GSX, it'll spin out fairly easy. Has reverse and neutral.
96' - 97' GSX is a fun ski, you'll get wet on it for sure, not as stable as the larger GTX hull, great as a 1 person ski. Unless the passenger is a small kid, it won't be comfortable with 2 on it. Big 15 gal tank, it'll go the distance, but can really cut up if you want to, digs in and pulls some g's. Carb 787 is like the 96'-97' GTX, easy to work on and maintain. Doesn't have a reverse or neutral, takes a little practice to dock with start and stopping. Little faster than the GTX, maybe 3-4 mph more on top end.
I love my 97' GTX, but I'm glad I added the 96' GSX as a compliment. Depending the mood or if someone goes out with me I'll take either. I prefer the 96' GSX because I can do everything, cruise a long time or cut up. Taking a passenger, like the wife, the 97' GTX.