Andy, you've said a couple of times that you don't have any interest or desire to do technical diving or even to dive doubles, but you have said that you want a class that will make you a "better" diver. What do you mean by that? Figuring out how you want to be "better" will help you choose a class.
If you want better buoyancy control, work on your trim, new propulsion techniques, refinement of emergency procedures, and to learn how to deploy an SMB, you could take GUE's Fundamentals class. You can do it in a single tank. You'll learn about gas management, some more about decompression, and a lot about diving as a team and about situational awareness. It does have equipment requirements, though.
Advanced Nitrox and Decompression is designed for people who want to do deeper diving requiring staged decompression. Although I'm sure you would learn quite a bit from such a class, I'm not sure how applicable a lot of it would be to the diving you are telling us you do, and want to continue to do.
Everybody got excited about this class versus that, but they seem to have missed your original parameters.
