My Duramax gets about the same as the others. I would look into a 3500 not much more money if at all and a lot more payload. Leaves options open down the road. I went from a 2500 Short Box 2 @ 3500 long box and have been very pleased. For me mileage was not nearly as big of an issue as the pure fun it is to pull with a diesel. It just feels great going 60 mph at 1400 rpms, up and down hills without changing gears. But then there are the issues that diesels have being a more complex motor with modern exhaust def requirements. Also I have read alot about potential costly fuel pump repair. Like 10k worth. I myself had def issues a couple years ago but the dealer took care of them and I haven't had any issues sense. But I do LOVE my Duramax.
Just my humble opinions