I think anything you will buy you will like.
All of the SJ's prior to 2008 have the same hull, so as far as handling all of them are the same. Not a lot of difference in the SJ's. The main difference is that in '94 the Sj changed from a 650 to a 701 motor. '96 they changed to the RN and kept the 701. The Rn does look nicer and I think there's a few more options for the RN like poles and etc but that's not life altering.
I don't own a superjet, but I think that can sum it up.
The Kawi SXR is better for racing but can be used for freestyle, just as the SJ is better for freestyle but can be raced.
I think any stand up you get you will enjoy. My stand up as my 1st. A '89 650SX. Cheap to get into the sport at $800. It has been great for me but other's say it's a POS. W/e, only problem I have had was one I caused with a bad gasket that caused water ingestion and blew my engine.
As for holding up in salt water, it will degrade as fast as any sit down or boat in the salt water. They take on more water so the entire engine compartment gets covered in salt. The key is to properly seal the electronics and hose down everything in WD40 to promote longevity.
Price ranges.... Depending on condition, year, mods and location...
650SX, 550SX $500-$1000
750SX/SXI/SXI PRO $1200-$2000
SXR $3500-$5500
SN SJ $800-$2200
RN SJ $2700-$5500 (Or as high as $8000 for heavily modded skis with after market hulls, engines or carbon fiber)
X2 $400-$1200
B1 (you don't wat the Blaster2) $1200-$1800
And don't listen to anyone who says take a loan out. Spend only what you can afford and expect maintance costs. Buy used.