... So I sent an email to Zax to really help us all learn!
In a nutshell the difference is that ProAnimator includes some very advanced tools that help you create animations much faster and much easier than if you used Invigorator Pro with AE's standard keyframing techniques.
With ProAnimator you can create very complicated animations in minutes. If you used AE's keyframes it would take you hours and hours of tedious labor to do what ProAnimator can do in a few clicks. ProAnimator frees a designer from the technicalities of keyframe-wrangling. It plays the animation in real-time and you can make changes while the animation is still playing. As a result you can change-change-change the animation until you get exactly the motion you want, modifying the equivalent of hundreds of keyframes with just a few clicks.
Standard keyframing is similar to building a house out of toothpicks. Everybody tries to build a big house once. But after that you start scaling back your ideas to fit the amount of time you have to work in and the amount of trouble you want to cause for yourself. ProAnimator helps you take the lid off your ideas. You can start thinking of big ideas again and ProAnimator will let you do them quickly and easily. If keyframing is like building a house out of toothpicks, ProAnimator is almost like having a magic genie which will do what you command.