500s, 600s, 700s and 750s (including their 5- or 6-string variations) all have the same electronics, hardware and necks (5pc jatoba/bubinga). The only difference is body wood:
- SR500: Mahogany
- SR530: Mahogany (and 5pc maple/rosewood neck -- okay, I lied, that one's different)
- SR600: Light ash
- SR700: Mahogany w/ figured maple top
- SR750: Mahogany w/ walnut top
There's also hardware color (but seriously, who cares about that?) and what variations are available. For example, they all come in 4- or 5-string versions, but if you want a 6-string the 506 and 706 are your only choices. Likewise, if you're a lefty, it's a 500L or 505L for you.
A left-handed 6-string bass, I hear you ask? Well... No.
EDIT: In the general case, I'd say you don't gain a lot, sound-wise, from getting a non-prestige SR above the 500. But then again, the price difference isn't much either -- Thomann says the 755 costs only 70 euros more than the 505, so whatever you think looks best... If I ever get a sixer, well, the 706 is damn sexy, I gotta admit.