I own both, 2 different types of rifles.
Ive owned a mini-14 for many years, Ive tried several scopes, scope mounts, ammo of all kinds, stocks, you name it. Its a fun rifle to shoot and not expect alot of accuracy. In fact with some real "accuracy nuts" helping me years ago we determined that in order to shoot decent groups with it you must have a very good powerful scope, and take about 1 shot every 10 minutes with perfect conditions at 100 meters.
There is no comparison between it and an AR-15 at all. The AR was designed as a civilian version of a precision shooting mil-spec battle rifle.
My AR is extremely accurate, at 100 meters with a 1X red dot scope I can make groups .5-1 inch easily, and can do it with rapid follow up shots, in fact the entire 30 rd mag can be fired easily into a 2 inch circle at 100 meters.
Other AR experience, with open A2 sights you can knock down human silouettes at 400 plus meters all day. With a 4X scope on that same AR you can knock down sillouettes at 650 meters.
The mini-14 is an inexpensive all purpose ranch rifle. Its accurate at 50 yards, I found it a waste of time to set the scope beyond that. The mini has a barrel length advantage over the AR carbine, but it doesnt make a bit of difference at all. My AR (I picked a 16" with a match heavy barrel) is great for follow up shots, Ive yet to heat the barrel up enough to really effect the accuracy.
The AR is lighter, more durable built, better durable design, far superior in quality and much more user friendly.