The only problem with a 650sx is that it is a 20 year old boat and often has been poorly maintained at some point in it's life. The same is true of the square nose superjet. I enjoyed my 650sx, but if I could go back in time, i would have spent the extra money on a newer boat. That's not a Superjet, not a 750SXI, not a SXR, just newer. If a 2004 650sx could be purchased in excellent condition, I may have considered one over my superjet (assuming the price was better), But there is no such thing as a 2004 650sx, thus the reason I own a superjet.
A 2000+ Superjet may cost you $2500-$3500 in good condition, but it will be newer and easier to find in good condition. The 650sx will be nearly impossible to find in good-excellent condition.
The 650sx is a fun boat. I enjoyed mine very much (1st ski, 44mm mikuni carb, ride plate, intake grate and prop) when I was riding it. But I often wasn't riding it, because it was an old worn out piece of crap that was always costing me time and money in repairs.
I like the 650sx. I'm considering pulling my X2 motor and building my 650sx. However, that's because it'll be almost all new. Even then I will still have electronics from 1994 begging for more problems.
Long story short. Buy something because it doesn't need work (much), because in the end, the amount of time the boat is ready to be ridden is the most important feature of all. I started with a 650sx and wish I hadn't, not because it was a 650sx, but because it was old and busted.