ProAnimator and Invigorator Pro share modeling and texturing features. Both can work as plug-ins to After Effects. ProAnimator also works as a stand-alone application which can make movies you may then import into any video editing applicationn.
The key difference is how ProAnimator and Invigorator deal with animation. Invigorator uses After Effects keyframes and is limited in the number of elements which can be independently animated -- these elements are called sets, which amount to invisible containers into which one or more objects may be placed. Being able to use after effects keyframes and animation controls to which those can be applied, in some circumstances, be very useful .. and something you can't do with ProAnimator's animation controls.
ProAnimator can animated an unlimited number of objects... either as related groups of objects in a single object track, or as individual objects in their own object tracks. (RAM is the only limiting factor) It can also create very complex motions very quickly and without keyframes. ProAnimator's behavior-based animation system is very, very powerful and allows you to create motions that would be difficult, if not impossible, in Invigorator.
Personally, I'd choose ProAnimator, if I had to choose one or the other. The motion controls are just that good. I have both and use both, but find myself turning to ProAnimator more often than not.