They're all good. The SR300 is a heap load of bass for the money, I think they're about £245 new. The 370, you're paying a little bit more for a nicer paint finish, the hardware and pickups are the same as the 300. I've an older 300, it's a great bass.
The 500 has better pickups, IMO. Slightly different hardware, but not much in that really. And it's made out of more expensive woods.
If they did the 300 with the Bart pickups from the 500...for about£3-350, that would be a knock out bass.