I've thought about getting an M1 Carbine for a long time, and though I'm well into the double digits in guns, I am still thinking about it. Why? It just doesn't seem like it serves a purpose for me.
The primary drawback to the .30 round is that nobody has bothered to develop decent loads for them. It is about as powerful as a .357 out of a rifle, so it is nothing to scoff at, but bullet design is still pretty rudimentary. Compare that to the 5.56, which has had decades of R&D put into making it a good killer, and you will see that your options are a lot greater on that end.
The upside for the carbine is that it is a very light kicker with little muzzle blast (especially compared to a 16" barrel on a 5.56), it has good sights to start with (Ruger is fixing this with new Minis) and, most of all, reliable hi-cap mags are available readily and on the cheap. 20 rounds is the most a Civilian is going to get from Ruger, and that is at a cost of $40 a piece. You can buy two 30 and two 15 round mags for the .30 for about that much. Also, loading the M1 Carbine is a more strightforwad proposition than the Mini series. It takes a little getting used to rocking the mags appropriately with the Mini.
Anyhow, aftermakret accessories abound for both. I think the .30 has a niche as a defensive rifle for small-framed women but that the Mini is a more potent weapon. The Carbine also has the advantage of being truly battle-tested. The Mini-14, to the best of my knowledge, has seen virtually no use in combat. No, police use is NOT combat.
If it is any indication of utility, the Israeli police still use and prefer the M1 Carbine for patrol duty. Their new rifle even takes the Carbine magazines!