Poser 8 will run on 64bit, but both the program executable and the render engine are 32bit.  It is a "clean slate" rebuild, utilizes multiple processor cores very well, and renders quickly.  It adds IDL.   It has a new GUI and library UI in particular.  The program startup is quicker than P7 or  Pro.
A fair percentage of people have reported bugs;  others, like myself, have not had problems.  Scan the forum for these issues.  SM is working on them for SR2.

Poser Pro's program executable is 32bit, but is has both 32bit and 64bit render engines.  Render in separate process (don't click the little camera-shaped render button) to engage the 64bit renderer.  It will run four threads.  The 64bit render engine allows for large pixel dimension renders of complex scenes with complicated materials.  PPro is stable and bug-free in my experience.

As I understand it, either program will be eligible for upgrade when Poser Pro 2010 is released.  I think it will fuse the clean slate leanness, multi-core utilization, new GUI system, and IDL of P8, with the 64bit render capacity of PPro.

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