Kinda depends on what sort of scenes you want to do and how much effort you are willing to put into the whole thing.

I personally use XSI/mentalray to render poser stuff and my own scenes. For me it's the most flexible solution. But people who do lot's of environments may need something like Vue or those doing interiors may prefer VRay. There's also the question of how seamless a connection you want with poser. If you just want to drop a scene and render then C4D is probably the best option.

Quality wise, all of them outdo poser. Even the free ones.