I believe the difference is that the Pro Series is " pro shop only" . The rest are not. (I may be wrong on that?)
As far as the Nitro goes...I have never personally shot one. I' m sure they are nice....but a little low on BH for my likings. I personally think the Primos is probably better. That seems to be a great setup all around. I currently shoot a Thunderbolt, and I love it. Go test drive a few and see what you like and what fits your needs.