You are looking at it wrong. It is not the trucks capacity to tow that is important. It is the trucks ability to STOP that is important. Second is suspension. THIRD is the durability of the transmission. Then power. With a 1500 series, towing short distance, 5000 is safe. long distance on the freeway, you are going to overheat the brakes and the transmission. If you want to tow 5000lbs or more down the highway for hours at a time, get a 2500. Not for the deisel, but for the suspension and braking capacity.
Trailer brakes are all fine, but at some point in the braking process, even just to slow for traffic, the disks on your 1500, towing a 5000lb trailer, are going to be dealing with over 11000 lbs of weight. The disks are going to get very hot. The pads are going to wear excessively. Get the 2500. Even a used one will be better than a new 1500 if you plan on doing a lot of traveling with a heavy trailer. Or get a tow beast. A 3500.