I've had several vehicles that had either the 5.3, 6.0, 6.2, 8.1 or the duramax and if you plan to tow a considerable distance I would recommend something with the 6.2 or the 8.1. I've towed a X45 with a 1/2 ton GMC crew cab pickup with the 5.3, while it will tow it, I would not recommend frequent towing with the half ton unless the distance is extremely short. I also towed the X45 with a tahoe with the 5.3 and I would rather tow with the pickup vs the tahoe.
I've towed a 197 with a 3/4 ton crew cab with 6.0, 8.1 and duramax and I will say that the 8.1 pulled the boat the best. The 3/4 ton with the 8.1 had 4.11 gears and the 3/4 ton with the duramax had 3.42 gears and even with the edge chip the diesel did not compare. You couldn't tell the boat was behind you with the 8.1 and 4.11 gears. The truck would only get about 12 miles to the gallon empty or 9 while towing. I was also not impressed with the mpg of the duramax (2005) regardless of the mode it was set to.
If you don't have to tow very far then the 5.3 would be fine and will get better mpg on an avg. The 6.0 would be a good medium duty but if you tow a long distance I would definately get the 8.1 or 6.2. I'm basing this on experience from towing trailers and boats from coast to coast. So, this is based on lots of interstate travel and in town commuting.