Got highs, mids and lows, all of which offer pros and cons, so I use whatever floats my boat on that particular day.
Having said that, with airsoft rifles, and even smgs, we don't have the accuracy, range or penetration of the real weaponry, and sometimes people don't feel a hit, so we typically need a few more rounds to get a hit than you would with the real weapon, especially in woodlands, where even a leaf or twig can stop a round, which if it was the real weapon firing, wouldn't make much difference at all. That's never that much of an issue for me, because I prefer firing on semi-auto most of the time, with aimed double or triple taps and would probably only select full auto if someone needed me to keep someone's head down whilst they ran for cover or some such, so I can cheerfully get by with low caps, but for those who like to auto fire more often, I would say that mid capacity mags probably offer an experience more akin to the real thing, when we take that lack of range, accuracy and penetration with airsoft weapons into account, i.e they offer enough ammo to compensate for the shortcomings of airsoft weapons, but not the stupidly overpowered ability to cap off 300 rounds from what is supposed to be an assault rifle with a thirty round mag and a barrel that would almost certainly be on fire if it had 60 rounds go through it in one burst.
In addition to that, I tend to regard having a speed loader in a vest pocket as the equivalent of carrying bandoliers or fast loading stripper clips (but without the weight fortunately), so even if I had just three or four low cap mags on me, I'd still basically have enough ammo to reload two or three of those, and that is definitely what I do with GBBs, because those mags can get pretty expensive, and unlike on the real battlefield, our GBB mags are heavy even when empty and are not really disposable either.
