Spend some time doing a search and reading the responses.
I am not an accomplished rifleman, but I have owned several of each. My experience seems to mirror much of what I read:
--the Mini is a quality built, reliable and competent rifle.
--the AR15 is an excellent rifle.
If you want to shoot fist sized groups at 100 yards, the Mini will do it.
If you want to shoot dime sized groups at 100 yards, the AR is the way to go.
If you want to shoot at ranges greater than 100 yards, the difference between the two grows exponentially.
In general, it's less painful to pay once, and get what you really want.