I OWN both. My Pro is the newer 2009 version. Both have adjustable gullets, btw. The twist on the Pro is wider - similar to the treeless I used to ride in. I loved the security the Isabell provided on the horse I was leasing - I didn't feel locked in and I was also using the larger knee blocks. The Isabell has the full equisuede and the Pro has the leather like at the bottom flap. The flaps are different sizes, I preferred the shape of the Isabell. The Isabell also have cushion in the knees rolls I think. Isabells almost always come w/ Cair, my Pro did also.
I think it comes down to personal preference. They are 2 totally different seats. I also got the Pro in an 18 and the Isabell in an 18 because they tend to run small.
The saddles are about a $250 difference purchased new.
Both mine are for sale because neither fit my new horse properly tho of the 2 I prefer the Isabell. If you're interested I currently have them on Ebay: eBay My World - shadowanne