I have both Poser Pro and Poser 8 concurrently installed on both Galadriel and pixie.  I generally try to render in P8 because of it's speed and IDL option. I tend toward complicated scenes, use complex materials, and often render at large pixel dimensions.  This often runs P8 out of memory, so in those cases I find myself using Poser Pro to do the "heavy lifting", since Pro can use all of a 64bit machine's RAM.

Overall, I would say that P8 has several quirks which need to be de-bugged (hopefully SR2 will do it), but it is a clean slate rebuild, and has considerable potential since it is relatively unburdened by "legacy luggage".

How much tolerance do you have for dealing with development (teething trouble) flaws?  Pro is fully worked up;  P8 still has some quirks (SM is working on them).

What about your usage habits - how memory hungry are your scenes?  If you tend toward the complex, remember that Pro has much greater capacity if you have a 64bit system with lots of RAM.

If render speed is important, or you want the benfits of the new IDL, only P8 has it.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5