A year ago, I was faced with the same decisions- after towing my 6500 lb. trailer for a few trips, mostly in the mountains, with my Ram 1500, I felt like that even tho it was rated to do so, it was really struggling at high RPM over the mountain passes, and just didn't feel like I was fully in control.
I sold it, bought a GMC Sierra 2500 HD Duramax, and could not be happier about the decision. Plenty of power uphill, but as important to me, the exhaust braking capability is a big plus on long downhills.
Another important part of the equation, IMO, when towing, is the transmission. The GM Allison tranny in the Duramax version is one of the best there is.
I think that as a general rule of thumb, that if you are routinely towing over 5000#, the 2500 series trucks are essential, with the diesels being icing on the cake.
