I'd suggest remus makes the best point relative to your choices! Computing power - I think this will cause you to rip hairs with any unbiased option. You need grunt!!!!!
That said I have a feeling as computing power grows as even quad core machines are cheaper and cheaper by the day and 8 core machines likely to be more common place, 64 bit is proving faster with maxwell for example. I think you need to consider this and future proof yourself!!! Are you going to buy a new machine in the future???
One other thing to consider is that if you are exporting from SU think about the poly count limits that currently dog SU as an exporter for rendering and look at apps that enable instancing and if possible external referencing of components!!!
I know maxwell aims to shortly enable the option to replace low poly components upon export with high poly MXS files (maxwell geometry) meaning trees and plants as an example created in apps such as Onyx or xfrog can be bought into maxwell studio and material edited and saved. This file can then be later referenced to a low poly proxy in SU!!! This will mean a great increase in realism in any output and a much smoother workflow in SU.
Think also about the available materials libraries and what is available! Vray has been around for a long time and there is tons of materials available, maxwell's material gallery now has thousands of materials available, Fry has really just kicked off so not so much available there.
I don't think these choices are easy as the workflows and resources are so varied! but mate if you find you can get any output fromm vray you are far more up the ladder than me as I couldn't understand one quarter of the options available to get good output.