There is no such thing as the best renderer, it all depends on what you want, what is in the scene and the amount of time you are willing to spend on rendering.

For any close-up work you are probably best off with Poser (Firefly), for any rendering from some distance (with mutiply figures) you are better off with Carrara (currently my renderer of choice) or Vue.

Getting a whole scene from Poser into Carrara is very easy, just open it. Depending on Native or Transposer (strongly advised) you may get everything in except for camera and complex materials. Remove all lights except for the spotlights (point light seem to loose their position, other lights are better defined with the lights available in Carrara), find the correct camera setting (camera's in Carrara are more proper then the Poser ones) and adjust complex materials (the materials in Carrara are just as powerfull, just different). In general the rendering in Carrara is quicker and of a better quality (depending on lights and materials) but with a complex scene it can still take long (just finished a high quality rendering taking 65 hours).

Getting a scene from Bryce into anything else is completely out of the question. Bryce uses its own format (not compatible with anything else). So is using Bryce for rendering (way too slow).