I have ridden all the stand ups from Yamaha and Kawasaki and owned most of them too at some point. For a beginner to the stand ups you will want to make sure you like it before you dump a ton of cash on one. They are so much work compared to a sit down and you will be exhausted after about half an hour your first time out.
1. JS300, JS440 and JS550 they are cheap but you will hate it as your first stand up. They are verry narrow and unstable as well as underpowered for the most part. When you need the power to pull you through a turn as a neewbe it will not be there. They just are not stable enough to learn on. Price is usually less than $1,000 for a good one.
2. JS650 1987-1990 use a small carb and side exhaust. Much more stable than a JS300-550 and still very cheap. The 650 is very reliable. Price is usually less than $1,000 for a good one.
3. JS650 1991-1992 same hull as older one but bigger and better carb and rear exhaust. Best bang for the buck. A clean one is usually close to $1,000
4. JS750 SX More stable and larger than the JS650 but low on power and has a hard time getting out if it's own way. Usually about $1,000 for a clean one.
5. JS750 SXI same as SX but twin carb and much better power out of the hole. Good ski if it can be found cheap. Usually $1,200 for a clean one.
6. JS750 SXI Pro. Best hull of the Kawasaki 750's with the same twin carb 750 as the SXI. Besk Kawasaki until the SX-R800 came out. Usually about $1,400 for a clean one but much harder to find.
7. SuperJet 650 Square Nose 1990-1993 Good hull but a little underpowered. Still expensive in California. $1,500+.
8. SuperJet 701 Square Nose 1994-1995 Larger 701 engine, single carb and square nose deck. Very stable ski and good power. The best Yamaha for the buck. Still $1,500+
9. SuperJet 701 Round Nose 1996-+ Same 701 but with twin carbs and new upper deck with round nose. This is the superjet that everybody wants. Very stable and good power especially on the bottom but runs out on top stock. Expect $2,000+ for just average ones.
The Kawasaki's main difference from the Yamaha's is how they turn and the power. The Kawasaki's like to turn leaned over on the inside with your butt dragging in the water. The yamaha's like to be kept flat and turn on your feet. In seadooo terms the Kawasaki's are like an HX that like to lean over and the Yamaha are like a X4 or GSX that like to be more flat in a turn. The other difference is the power. The Kawasaki has more top end and less bottom end and will usually be a little faster and the Yamaha's have more bottom and less top, more grunt out of the hole.
It all depends on your riding style. I have always been faster on the Kawasaki's as that is what I learned on and got used to the lean in turning.