I'm not too sure what your meaning saying you want it sharp? Up the anti alias settings, also if your rendering in one size and then uploading it to a site that uses another resolution your going to get some compression

I wouldnt recomment getting another software package, the software render can produce some amazing results and I think everyone knows what mental ray can do.

Theres some books you can get on rendering, I would save your cash and get one of them, learn what maya can offer before going with other softwares.

Ps - Just thought are you asking on actual render softwares or compositing softwares???