Ok, so which is the better truck, I currently have an 06 2500 qc short bed 2wd with a g56 in it. It is an awesome truck IMO, but would be better off with a 3500. There is absolutely nothing wrong with my truck at the moment...113k miles....
Are there many bad things with an 03's drivetrain, lookin at a 3500 srw quad cab short bed with a nv5600 and electric t-case and 140k. It also has a pac-brake on it.
Do the 03's have a cat, and what are their problems....would I be better off with the 06 or getting the 03??

 

I would keep the 06. You know the 06 has no problems and the 03 could have hidden issues. If you need to have a heaver load capacity then add air bags to the rear axel. The only difference in a 2500 and the 3500 SRW is the rear springs and of course the clearance lights.

There is no cat on the 03's. The lift pump on the filter canister is a weak pump but is easily replaced. The alternator stamped bracket has probably been replaced with the cast bracket. I have replaced both front axel u-joints. Recently replaced the water pump. Other than that mine has given me good service.

 

I would say the advantage to the 03 truck, if there aren't any issues that you don't know about, are the exhaust brake and it would likely get better mileage.
Other than that I think papaduck hit it on the head.....

 

The above 2 posts are right on the money.

I would also keep the 06. Worst case is you could even do a dually conversion and keep the truck you know 100% about.

 

sounds like I keep the 06 and pony up on some suspension mods and add an exhaust brake to it....

 

Good choice! Probably much better and cheaper in the long run