As far as i know the Vray SSS is faster and more robust than mentalrays, but i have no sources for backup. The Vray docs are all on chaosgroup's website, under the "support" tab
Both are technically totally capable of making photo realistic renders. But getting to that point is very challenging. I have more experience with mental ray, but iv seen more 3d portrait artist types using Vray. Maya has pretty good built in documentation for Mentalray. Not sure where Vray docs would be. Don't really have…