I can't remember who said that they purpose of the render engine is to translate your settings into picture. So then knowing the limits of your tools will be key (stuntman driving a jalopy vs. teen driving a Porsche).

I'm no expert but Firefly can look great once the light parameters are adjusted. It goes a long way from removing that characterisitc Poser look. Unfortunately Poser / Firefly is not the most efficient combo.

Several versions of Poser export RIB files, RIBs being part of the Renderman specification. There are a few free Renderman compliant render engines that you could experiment with and the jump from Firefly is minimal since it too is Renderman compliant (to a point).

I love mental ray. It's powerful, physically accurate and very fast. It is complex and requires lots of reading and research but it's well worth it. As mentioned earlier, mental ray's SSS is phenomenal.

I'm still learning the Vue renderer but I think its strength lies in massive datasets, and V6I's render engine is quite a bit faster than V5I. I am having difficulty getting zero influence from the atmosphere, which contributes to that "Vue look". No big deal though. Good luck.