This is a superjet and freeride dominate forum. The answer here from most will be to "buy the superjet". It doesn't matter what you ask about, they will tell you to buy the SJ.
You have to decide what you want to do. If you want to race, get the 750.
If you want to do tricks, get the SJ.
If you want to race and ride surf, the 750 will work out ok for you.
I personally wouldn't spend the money to change from a 650sx to a 650SN. To me, that is a waste of money. The SJ is a better hull all around than the 650sx but it's not about what is a better ride, it's about $$ at your age. I haven't forgotten what it was like.
Any running ski is going to pull a few bucks, so the difference b/t a 650sn and a 701 61x SN wont be much, and if you can find a 701 62t sn, even better, although harder to find, they don't sell for much more. It's worth saving the extra money and buying the 62T 701 if at all possible. At the very least, the 61x 701 since almost everything is the same except the intake.
The 701 motor is going to be an expensive upgrade into that hull, and the 650 motor is going to sell for nill.
If you were asking about the SJ vs the SXR, we could have some discussion, the SXR is an awesome boat if you want to run buoys. The 750 vs the SN is a little less of a debate. I'd take my SJ over the 750 even for racing. I just didn't like the 750 when I rode it. Maybe I'd feel different now as I've grown into the sport, but the 750 felt like a tank in the nose and I didn't like it way back when.
The RN SJ is going to be a waste of money for you, since you pay a huge step in price for that hull. The benefit of buying the RN SJ is they condition of the parts, less likely to need repairs, and in the longer term, a better investment. I wouldn't wait to save enough money to pay double for a RN over a SN.
Keep saving your money, and buy a very clean 701 SN SJ when it becomes available.
If for some reason you want to wait long enough to buy a RN SJ vs SXR, for racing, the SXR is hands down the winner. Otherwise, the SJ is better for free ride, surf and flatwater.