I've owned Mini-14's, AR-15's, M1 carbines, and M1-A's. The Mini-14 feels flimsy, has a lot of loose parts, is built cheap like using folded sheetmetal for a hammer, a backwards gas system that can seize up, and lousy sights. There's no comparison between it and the military-derived rifles.
I'm going to shoot some 3-gun soon and my plan was to use my M1 carbine. It doesn't matter if some people like the ammo or not, it makes Minor power factor and that's all I care about. There are some disadvantages. I don't know of any 10 round magazines for it and surely wouldn't advocate breaking of the law on the internet to get some. I already have a good stockpile so it's not a problem for me. However, the 15 round magazines put me at a disadvantage compared t the AR's. I've tried various 30 rounders but they're not very reliable. Also it's not easy to add optics. An M1-A would work better but would cost a lot more. I think you can get 10 round mags.
Forget the pump. It will suck when shooting prone, which you can bet on having to do in a 3-gun.
I don't know why someone doesn't design and make an AR lower with a tradional pistol grip. They would sell the piss out of them. It would be considerably different than an AR-15 so it shouldn't fall under the law banning AR-15's and their clones.