I have a Little Gracie II. Honestly, looking at the two of them on Grace's site, there's very little that is different. The GMQ Pro will let you do king sized quilts, and has the height adjustable legs, but other than that, it looks exactly like what I have. If the LG II is significantly cheaper and you don't plan to do king-sized quilts, then I'd go with that, myself.
If the used frame that you end up buying does not have the carriage upgrade (it's metal, versus the wooden carriage that is the default for both frames), then I would highly advise getting it. It's a much smoother ride.