I don't use Dollar all that often, probably the same as you only when they have a better deal than
Alamo.
Some differences that I recall are:
1. They do not have kiosk, unless they have added them recently.
2. You do not get to choose your car as you do at Alamo, you will be given the keys and it's location in the garage, to a particular car when you leave the counter. That's not to say you can't change it after seeing it, but you would have to go to the desk in the garage and have to have them change the car. Never did it so I don't know if it's like starting over.
3. As far as my experience with Dollar goes, I have never had a problem with them. Some have reported that some of their counter reps can be hard sell on the insurance, but I never experienced it.
I was about to use Dollar for my upcoming trip in Nov until Alamo came out with a better deal, so I'm now back with Alamo, but if Dollar drops I'll be back with them. It's all about the price to me, I'm not hung up on what kind of a car it is or what it looks like, it's just transportation to me. I'm not buying it, just borrowing it.