I have a Pfaff PowerQuilter P3 with Quilt Artist 2. I have used a Gammill as well. I can't remember it's name, but it was quite large (the lady I rented from also had a smaller Gammill that was 18 inches). Both were effective machines, but there are a few things I like better about my Pfaff. First, I like that the computer is part of the machine. With the Gammill, there was a lap top sitting near by and that's where the computer layout of a design was tweeked. This may not be an issue for some, but I like everything as efficient and stream lined as possible. Second, the Gammill had tension issues regularly, but in fairness, I've only done a handful of quilts on my Pfaff whereas the Gammill was probably used every day all day. That may have been a factor in the tension issues. Second and most important to me, I purchased my Pfaff from my favorite LQS. They provide excellent customer service. I've been receiving weekly lessons to make sure I know all the things I can do with my machine. I can call them when I run into a problem and they will talk me through how to resolve it. This may or may not be important to you however.
To sum it up, if you are getting a good deal and you are able to learn on your own (something I don't do as well), either machine, if well cared for prior to your purchase, would most likely make you happy. How exciting for you!