Are 20-rounders more reliable than 30-rounders? If not, what advantages do you get with a 20-rounder?
I don’t believe they are intrinsically more reliable. The main advantage is weight, and length if firing from a prone position, or off of a benchrest, or short bipod.
A good brand of magazine will have equal reliability, but other variables may dictate which capacity you prefer. If weight and a 30-round footprint do not impede mission parameters, the Surefire 60 Round Magazine is pretty awesome. Feeds reliably and does not introduce any asymmetry into the magazine equation.
“20 rounders are better because you can get your head that much farther down behind cover” 
All joking aside
In reality I think there are times when the lower profile offered by a 20 rounder does come in handy. I always carry a couple when going to the range to zero an AR. I have also known guys who load special ammo in their 20 rounders so that they can distinguish what is what on the fly.
I own many 20 and 30 round Magpul mags, and often keep the ‘special’ rounds in the 20’s. My platform also employs a Magpul PRS w/ Monopod and a GG&G Bipod up front, so I can level it out and still have clearance for a 30 or Surefire 60 rd. You should own them all. The 20’s come in handy when shooting off a ruck or similar, and at that point I would probably prefer what’s loaded into the 20’s as well.
I do the same as already mentioned, I keep the good ammo in the 20 rounders and the training ammo in the 30’s so I know which mags to grab when shooting for groups. The 20’s are a must if mounted in my benchrest, 30’s dont fit. I can’t say if one is more reliable than the other, no problems to report here.
308!
SPR + bipod = GOTTA have it!
All my 30s are just as reliable as my few 20s.
I like 30s for everything except carrying in my truck or the rare (very rare) occasion I decide to shoot from a bench.
30s are best for shooting prone, but most of the time I wish they were a little longer because there always seems to be something in the way when I shoot prone…
.
I have had and have read about failures to lock back after the last round with P-Mag 20’s. Haven’t experienced or read about this problem with USGI 20’s, USGI 30’s or P-Mag 30’s.
I always carry a couple 20’s to shoot from the bench.
To me, 30’s are easier to handle than 20’s. To some people, the ease of loading into your gun might be a reliability issue??
But maybe it’s just me…
.
One advantage I have found to the 20 round magazine is the ability to lay prone, and get “lower” and really kiss the dirt with the gun. Obviously they stick out less, giving the gun a smaller profile and allow the shooter to get lower and in my opinion offer a more stable pronated shooting platform.
Drawback-ammunition capacity. When you are in a gunfight or going to one, you want all the ammo you can get.
I have had failures to lock back on a 20 round PMAG (marked mag#9 by me). The same carbine locked back every time on a metal Colt mag, so I am assuming the PMAG 20 had an issue. I was told by gents on here that some 20 round PMAGs do this, it has to do with the follower or the way it engages the bolt face. Who knows.
Thats my 2 cents. I am looking forward to trying the new PMAGS coming out soon.
They’re priceless when shooting 300 yards and beyond from prone. Even off my pack, a 30 is annoying as hell.
What’s more reliable?
A screw driver or a wrench?
All the same to me. One is just for doing some things a little different than the other.
What 20 round magazines are you asking about?
USGI
Magpul
Colt
NHTMG
Okay
There’s a good number of manufacturers that make them. Some are good some aren’t. Which 30 round mags are you comparing them against? Which manufacturer?
Personally, my Colt 20 round USGI mags are as reliable as my NHTMG 30 round USGI mags. Meaning one type mag doesn’t do anything better or worse than the other.
4 of those are the same thing. 
unless you’re talking about the old USGI aluminum followers… in which case the last 3 are the same thing.
Well I was more or less trying to get the OP’s angle, as so to speak.
But I’m pickin up what you’re throwin down…
Maybe it would be more fair/accurate to say ‘20 round vs. 28 round magazines?’
Thus far, I can’t say I’ve noticed any more reliability between USGI versions of either, so long as the USGI isn’t loaded with more than 28 rounds.
Magpul 30 rounder mags have not given me any issues so far, either. (Never tried their 20.)
20rd mags are nice for prone and bench shooting. Don’t worry about reliability if you just buy PMAGS, just load them and go.
Not saying this is the op’s situation, but in some places (like where I live), Pmags are not an option for anything over 10 rounds. We have no choice but to use pre-ban USGI type mags, as those are the only ones for the most part that were made prior to the AWB.
That sucks.