I drove a midsize GMC Canyon for 6 years, but in October of last year, sold it and bought a full-size Sierra.
Mid-Size
-Smaller, and lighter makes it feel more fun / sporty around town. Parking is easier.
-Off road, it fits where bigger trucks do not, and gives you more breathing room on trails where big trucks just barely fit.
-Gets the job done towing. Smaller engines have to rev high to make the power they need, but the power is there.
-I had a 6ft bed, so the truck was still pretty useful for truck stuff. I did use all the space on some occasions, but never felt like it was limiting me.
Full-Size
-Around town, the truck feels big and heavy. Way overkill for a trip to the grocerey store or the mall. It works, but sometimes you may need to park a little further from the store entrance.
-Excellent long roadtrip vehicle. The improvements in driver / passenger space, and interior storage are much appreciated.
-Off road? I'm not even going to try taking this thing off road. Sure, I'll take it down rutted dirt paths, and rough access roads, but I won't be trying to follow Jeeps over obstacles. The Rebel would be a bit better off than my Sierra Elevation, but still no Wrangler killer.
-Towing is an absolute breeze. The V8 really feels like it has power to spare when towing 2-3000 lb. Tow / haul mode, with automatic grade braking is a treat in the mountains.
-Fuel economy is about the same as my old Canyon, since I had lifted it and put larger all terrain tires on it, but have left the Sierra stock with street tires.
-The bigger bed makes packing for camping trips easier, and opens up even more options.