AR's are kinda like LEGO's for adults. You can do/attach almost anything with them.

Uppers come in a LOT of calibers. The only real constraint on the calibers is as long as the overall cartridge length is no more than a .223 Remington you can use a standard lower and they are accurate.

Direct gas impingement is standard or you can get a piston driven one.

Magazines are (generally) everywhere and cheaper than Mini 14 mags.

The tactical Mini 14 I don't have any experience with. The standard Mini 14 and the Ranch Rifle (which I have) are as reliable as a brick and they are piston driven. The Mini's are not generally as accurate as AR's.

One other thing about the Mini 14's - you do NOT want to crack a round off at a coyote or any other critter for that matter, out the driver's window of your truck unless you want the inside of your windshield to have chunks missing or cracked. They throw brass like a mofo.