which format to use mpeg or mp4 when rendering for bluray; is 25mbs mpeg better quality than 15mbs mp4?

and what about BluRay players compatibility playing back mpeg and mp4?

Don't worry about compatibility. Any decent player should handle both MPEG-2 and h.264. I use MPEG because I encode from PP with Debugmode and TMPGEnc.

Your other question is a little dodgier. I don't think your two data rates are a fair comparison. The numbers I've seen on rough data rate equivalent for the two are not 5:3, as you state. I don't know for certain but I'd guess that the MPEG quality would be better given your example.

There are other variables aside from data rate that must be mashed to come up with a viable comparison. In my view, it gets too complex to mangle the math and factor in the aesthetic considerations to be valuable. Plus, I hate math and am bad at it.

I prefer to go with anecdotal evidence which indicates to me that there's no real perceivable difference between the two codecs. Someone with a shed load of facts might be able to relegate my conclusion to the bin straightaway, but I probably wouldn't understand what they were saying anyway.