20-round mags are great when you're firing from a prone position since they don't stick out far enough to hit the ground. When I was in the army we often got 20-rounders when we went to the range but never saw them any other time. It was strictly 30-round mags for everything
except when we qualified.
I have a Beta C-Mag so I don't have to change mags very often.
I have 2 original Colt 20-round mags, with David Tubb's SpeedLock 10% extra power chrome silicon replacement springs. They're temporarily blocked off at 5 rounds each to make them legal for hunting. They're clamped together with a Choate "Clip Connector", there words, not mine. It's made for 30-round mags, but I modified it to fit the 20-round mags, for fast reloads.
I also have 1 Brownells 30-round mag with a chrome silicon spring, and 8 30-round GI mags. I replaced the original black followers with green anti-tilt followers in all the GI mags.
All 9 30-round mags and the 2 20-round mags are in 4 army surplus magazines pouches that I cut the grenade pockets off of, since I don't carry grenades. A rubber ball works as a spacer to keep the 20-rounders from falling to the bottom of the pouch.

If the SHTF I can grab my army surplus web gear at a moment's notice and "head for the hills".
CS (chrome silicon) springs are supposed to be a lot better than SS (stainless steel). My understanding of the way CS springs work is that you load the mag and it takes a slight set and that's the end of the story. But when you load a mag with a SS spring it will continue to take a set and keep gettting weaker over time. Brownells simply says, "Choose from a mil-spec stainless steel spring or our new, high lubricity, dry-lube-coated chrome silicon spring that’s corrosion-resistant certified to 500 hour salt spray standards, and less prone to spring 'set'.” They're both the same exact price so there's no reason at all to get SS springs.
http://www.brownells.com/aspx/NS/GunTech/NewsletterArchive.aspx?x=v&p=0&t=1&i=884