Music Man have flat fingerboards. Well, at last the 3 I have owned all did.
With the Sterling and Stingray, you can't say which is the 'better' bass, they are simply different.
For example, the Stingray will offer a more warm, old school tone, whilst the Sterling offers more aggressive, more mid range punch. The Stingray is somewhat scooped in the mids.
The Pickuips and Preamps are different, but you will get the classic music man vibe with either bass. I love them all!
Also, the Sterling offers a smaller neck and smaller offset body and the Stingray is quite a lot fatter. This doesn't neccesarily mean the sterling is lighter than the ray, as wood varies obviously.
Edit: Just thought I would say...Musc Man are very highlty regarded when it comes to 5 string basses. You really can't go wrong. Personally however, I would get the Bongo 5!