One fact which can not be overlooked and which is a very big difference between the two programs is that Invigorator uses KEYFRAMES inside of AE to animate the model (logos for instance), Proanimator does not use keyframes to animate, so the animation is done in proanimator with POSES (not keyframes). Both programs though can use AE cameras to fly around the model etc.
The proanimator poses are very easy to use and you can make very complex animation by just modifying one of the presets. You can make all letters of a 3d logo move indepently etc.
To do this in Invigorator (animating letters separatly) can be way more time consuming as you will have to keyframe every move of each letter separate, incl. motion paths ease ins & outs etc. But then again, if you choose to do this you can keyframe more precise movements then in proanimator.
For logo animation (where letters move separatly) I would use proanimator, for single 3d objects I would use invigorator, or logos where the word as one moves (not separae letters) I would also use invigorator.
I also would like to mention that if you know how to work with keyframe in AE the working inside of proanimtor with the pose system should not be a problem, I actually find the pose system much more users fiendly then keyframing.
Best would be, GET BOTH! They are great!
Regards,
Adeeb