I have an 2005 Chevy 3500 with allison and Duramax. Been great, except for doing head gaskets at around 140000 miles. 2005 was the first year without the injectors inside, it's called the LLY. The difference between the 2500 and 3500 is springs and I think the frame is a little beefier, same brakes etc.
When I was looking in 2005 the fords were essentially the same, usually the long beds were the 350's.
There's some good input here, but when you buy used, the market will dictate a lot of what you get. Happy hunting.