The Mini comes cheaper than an AR, breaks down to clean simpler and easier.
The AR is indeed more accurate, but can also be more problematic if you don't clean it after every use.
I asked one the guys who has won the Texas State rifle championship how often he cleans his AR match rifle. He said "never". I'm sure he was kidding somewhat, but I've noticed my AR takes a lot less to make it happy over my M1 Garand. You do need to pay attention to lubing it correctly and I would make sure the chamber is clean. We cleaned them a lot more when I was in the Navy, but we were doing a lot of training exercises with blanks and they definitely dirty a rifle more than shooting live ammo.