Since you own Poser 6, the upgrade to Poser 7 would be $ 129.99 and to Poser 7 Pro Base $ 149.99. For the difference of $ 20 I would advise taking the Pro version. No need for you to buy the full version of Pro. For people not owning a version of Poser 6 my advise would be to buy Poser 7, the Pro version (base or full) is too expensive.

For Carrara you need to keep a version of Poser 6 installed if you use TransPoser (dynamic cloth only).