My best guess is that either
My best guess is that either engine can handle this load with comfort and ease, however, I see a compromise for each in real world use at the loads you're referring to. I can't speak to the transmission issues or durability or vibration of GM-specific products.
The compromise for the V6 gas guzzler is driving refinement while towing and fuel economy. I'd guess you'd continue to get poor mpg due to the torque curve of the Colorado / Canyon with this engine/transmission setup, because when towing, it will continually shift down to get the torque and hp it needs for the load conquering hills and such, because it's only at high RPM that a naturally-aspired gas engine gets power and torque, which you've discovered with regards to your Toyota. The new Colorado/Canyon will handle it with more refinement, but I doubt with much better mpg than what you're seeing right now, but that's just a guess.
The diesel, on the other hand, has all the torque it needs at virtually all usable RPM ranges, and therefore, towing with this truck, with this engine, should result in far better mpg than you've been seeing. It will lose mpg with the load of course, but since the power train will find the best power/torque combination at a far lower RPM; and the fact that it's compression ignition with lots of air versus fuel for combustion (a trick a gas engine can't do even if it's turbo charged); it should far exceed the V6 with respect to towing fuel economy. However, the compromise with the diesel is that once you reach peak torque at or about 2000 RPM, the power will continue to come up as you rev just like a gas engine, but well before you reach the peak horsepower of 181, the torque will start disintegrating, and therefore, you don't keep gaining performance up the RPM range. What does this mean? Well, for a 4000 lb+ truck and a 3300 lb trailer, 181 peak horsepower is a little lacking if engine responsiveness and off-the-line performance is important to you. If for instance, you're on a hill, on the highway, with your trailer and load, and you're traveling 65 mph and you're turning 2500 RPM; when you push on the accelerator to get more out of it, it's likely not going to happen; at least not much. Steady speed--yes. Responsiveness and accelerating ability to pass on a hill with your load, is not going to be that good compared to the V 6 which has much more horsepower. However, when the V6 does this passing trick, it'll probably be getting about 4 mpg in the process, whereas the diesel doing its best, in the same situation, won't respond as well, but will be achieving probably three times that mpg, in the same situation.