My take (and I've played countless Musicman basses)....
1. If you really want a Musicman, try a couple out, and if you still want one, get the real deal. The cheaper series of basses they make are great for the money, but you'll always end up wanting the real deal.
2. USA Stingray vs USA Sterling - Stingray has the classic tone, slightly bigger neck (not so much P bass but almost), chrome control plate due to front loaded electronics, etc.
USA Sterling is smaller, sleeker, Jazz-type neck but IMO better than a Jazz, the pickups are in the same spot but the magnet is ceramic, so it sounds more aggressive in the mids...they really do bark and cut through the mix like crazy!
Similar EQ's.
I've owned 90s Rays and modern Rays from recent production, and I have to say they just keep on improving the basses with tiny tweaks (eg the compensated nut). Fit/finish is superb on all of mine.
PS - NOTHING, and I mean NOTHING sounds like a Musicman bass properly unless its a Musicman.