I have had both trucks and have towed my same trailer with both.
First had an 09 GMC 2500HD, 6.0 Vortec, CC, 4x4, 3.73 gears. It towed fine for what i needed.
Then went to an 08 GMC diesel, got rid of it after 1.5 years due to operating costs and was overkill for what i needed.
I now have a 14 Ram 2500HD, 6.4 Hemi, CC, 4x4, 3.73 gears. It seems to tow much better than the 6.0. the extra power is nice and i can actually comfortably use the cruise control.
If i were to purchase again, i'd definitely go with the Ram, but get the 4.10's.
Honestly, you cannot go wrong with either vehicle, just drive both and decide what you like best.