You can find a comparison here:

http://poser.smithmicro.com/comparison.html

If you create content or wish to learn how in the near future then Poser Pro 2012 is probably a necessity. It is also 64 bit (but also 32 bit for those still using that).

For a newcomer who is still learning 3D or just tinkering then Poser 9 may be enough. Check the product comparison chart and see what is vital to you.