I'll stick with 20 rounders.
1.) they're reliable, the #1 issue in a battle weapon
2.) they enable me to get closer to the ground, 30 and 40 rounders get in the way.
I also prefer the L1A1, as you can maintain your firing grip/shoulder weld, and reload with just the left hand.
On the M1 commentaries: A good hand at loading the M1 can sustain pretty much the same rate of fire as someone with 30 round detachable mags, an average hand can keep up with a man with 20 round detachable mags. The biggest mistake hoodlums will make is to underestimate the threat from an M1 weilding man.
Also, if there are 5-10 or 10-15 attackers, they aren't just going to keep advancing as the frist 8 are cut down like a video game. They'll be looking for cover, giving the M1 (or any other weapon) man time to reload. One should avoid CQB in a SHTF scenario, most likely ALL parties will take at least one hit, and that's unacceptable.