Personally I like user set, but depends on where and how I'm driving. I don't have the best brakes for towing, it's known for the truck and recommended to have trailer brakes over 1500lbs, this on a stock truck, capable of towing 5000lbs (rated 3500lbs with the manual gearbox). I have overheated the brakes on the truck before doing a lot of city. This was before I had trailer brakes, so I worry that proportional will cause the same issue since it doesn't like to brake enough. I like to run with a user set brakes around 3.5 (out of 10) to have the trailer always do it's part for it's own weight. The off-road trip we did back in spring I was around 5 in user-set if I remember right. On long highway runs I prefer proportional with a higher setting in case of an emergency stop. I had an emergency stop downhill this summer. I don't remember what mode I was in, but I want to say I was in proportional and while I stopped in time, I felt like it wasn't doing enough and I didn't think to crank up the dial. I still need more road time and testing with the controller. It's my first brake controller.
I cheated and did a faux-pas back in October. My truck was *very* due to get the brakes done. Despite having the parts, I had *zero* time to do them. Brake performance got so poor that I drove with the trailer and the trailer brakes set to 8 to compensate.