This is an often asked question. Heres a feel for what opinions exist.
http://forums.cgsociety.org/archive/.../t-373784.html
Personally i began using mental ray, but later moved over to vray because it had better online community support, and included vrmeshes, an awesome feature for rendering massive poly counts, ie trees, in sensible frame times. Note that the newest version of mental ray now does the same, apparently.
The gnomon tutorials are an awesome place to start getting to grips with vray.
http://www.thegnomonworkshop.com/dvds/cni01.html
http://www.thegnomonworkshop.com/dvds/cni02.html