Re: towing compacity of gmc sierra 2500hd
I would agree with what has been said above, if you don't really need the diesel truck, don't get it. Unless you really want it of course

I've got an '07 Chevy Silverado with the Duramax (LBZ) and Allison 1000 6Spd, and have been super happy with it. I'm just breakin her in @ 115k miles now. She runs like a champ. But, you do have 3 GALLON oil changes and fuel filters you will need to contend with on a regular basis.
Here in California, Unleaded and Diesel prices are running pretty much parallel for the most part, but I have been seeing diesel come down quite a bit (about $0.40 cheaper per gallon than premium here, compared to the last couple of years where premium and diesel were almost the same price!). In any case, I do a ton of hauling and towing (I'm a mechanical contractor), that said.. I will never buy another gas truck, unless it was a small/midsize to run around town in. You have to be realistic with yourself and what your uses for the truck will be, and really determine if you need it, and want to spend the extra dough on the diesel.
I used to think it was a "wash", and maybe that was the case back when diesel was $1.29 a gallon. Not the case anymore.