I will still use MentalRay when I have too while I slowly let the knowledge i've invested in it for linear workflow, deex render passes etc etc die.
The framebuffers / renderpasses from Vray are a freaking godsend. I am currently using the evaluation copy of Vray. But I am pretty damned sure I am going to make this purchase.
Last thing to look into is what to use for my render management. It will be a little trickier as I need to keep cost low. But still!
Vray is love.