Actually, I'm going to buck the trend and go with a Beta...
If you've lubed it right, there's no such thing as a jam. After 30 rounds, I'll be kind and say it's 4 seconds to eject mag, rip new one from pouch, seat in rifle, and close bolt. If you have a magpul/paracord and duct tape, lower it to 3. This is if the pouch is already open. That's at least 3 seconds that an enemy can pop up and take their shot, while you're behind cover or concealment. 3-4 seconds is a pretty long time when someone's shooting at you.
I'd rather swallow the 20 rounds and save that 15 seconds. I've dropped mine (not fully loaded), no breakage, and it functioned fine. Ok, so it wasn't fully loaded and it wasn't on concrete, but...
You can also get prone easier with a Beta. Prone + more ammo than a 30 (or 20) = major good vibes. Especially if you're in a fixed position with a good field of view and fire.
The heavier weight, for me, also aids in keeping the muzzle down.
Then again, I am not privy to FA stuff, so..[:(]
IMO, IMO alone..