I haven't pulled the Jeep with a 4.3, but a few years ago I had a GMC Safari with the 4.3 and used it to pull my travel trailer a couple times. The trailer was within the weight limit of the minivan (6,200lbs loaded out), and the van had enough power for flat lands, but not enough brakes or gearing in my opinion despite what the owner's manual SAID the thing would pull. With a Sierra, you shouldn't have to worry about the brakes. You are going to see a lot of high RPM pulls on hills though, and that's rough on a truck over the long haul.
I would recommend always going one step bigger than you think you need. If the 4.3 will do it, go with the 5.3. The gas mileage improvement of a 4.3 over the 5.3 really isn't that much I don't think. For no more than you'll pull, the 6.0 probably isn't a good answer because your gas mileage will suffer.
Something else to consider, and I hate even bringing it up because I can't stand Furds, but that new Ecoboost motor is something else. Not sure what they've done with transmission, gearing, brakes, etc. though.