I've found mid caps 90-120'ish rounds to be the most versatile.
In "fun" skirmishes where there's no ammo limit and quick games, you can load them up and blast away. Once you're over the spray and pray phase...a good load out of mid-caps will go a long, long way.
For games where there's ammo limits (i.e. real cap loadouts), you can load your midcaps to 33rnds (the extras are for the loss of bbs that don't make it up through the hopup chamber). If you continually mash real cap mags to max capacity...the springs tend to loose their spring and don't feed the last couple of bbs reliably (at least the ones I had long ago for the M4 and MP5 did that).
So for a 10 mid-cap mag load out in a fun game you'll end up with 900-1200 rounds on you (which is pretty non-reality...but those are for fun games after all).
HiCaps are laughable....it sounds like you're running around with a half empty cookie tin filled with marbles (or more to the point a metal tin half filled with bbs). They make such a racked that it's just plain annoying. And...you never get to simulate the "Oh-shit, got to fumble a reload in the middle of a firefight"...which is a lot of fun.
Best of luck,