Patman,
I spend most of my online time (which is a lot) on detailing message boards b/c I'm wanting to start my own detailing business. Here's what I can tell you from what I've read on those particular products...
Meg's says that NXT is one of their longest lasting waxes. It's actually more of a sealant therefore it lasts longer than a carnauba wax. Many people report that NXT doesn't last. NXT is known to loose it's "look" very quickly which in turn is why people say it doesn't last very long. I don't know how you determine how long a wax lasts on your car, but for me, as long as it's beading I know it's protected. Maybe it's lasting longer than you think...but maybe not.
As far as GC is concerned, it's obviously a carnauba wax so it is not going to last as long. Meg's uses some sort of polishing oils in it which leads to a great deep wet look on dark finishes. One of my friends who has a black Trailblazer raves about GC and how good it looks on his truck.
As Brett mentioned, #26 High-Tech Yellow Wax is great stuff. I've not used NXT or Gold Class, but I have used #26. I found it very easy to work with (using a Porter Cable buffer). Very easy to buff off after applying.
Not sure what you know about all this, but sealants are kinda new and are known for longer-lasting protection. They also give a glassy look to them where as carnauba wax gives a deep, wet look (most people prefer the look of carnauba). However, carnauba doesn't last as long. Many people who want the best of both worlds just use both. Use a sealant like NXT first, then top it with a carnauba like GC or #26 after that. This way you have the lasting protection of a sealant and the great look of a carnauba.
Check out
http://meguiarsonline.com/ . Meg's forum has a TON of very detailed information on every single product that they offer.