Before I started playing with AA I was definitly a high-cap user.
Why, because it's a lot easier to wind a wheel then take a mag out, reload, and pray you don't get hit.
However now that I've had the chance to expereience a legit game more along the lines of Milsim, I PREFER mid-caps so much more over hi-caps.
The only reason why I don't use standards is because my load-out doesn't have a lot of mag pouches, not to mention I prefer the lower-profile rig so carrying fewer Mags that hold more rounds is the perfect set-up for my style.
As for midcaps vs. hi-caps.
Mid-caps feed much cleaner and more reliably. They don't skip rounds especially on guns that use a higher voltage batter or have certain gear-box mods to increase RPMs.
Mid-caps don't rattle and they give you the excitement of reloading (Seriously changes the entire game play making it a lot more fun)(There's a certain feeling about tagging someone and popping a new mag in you just don't get from hi-caps).
Mid-caps are fairly affordable in price as far as Star or Mag mags go (I prefer MAG) so it's affordable to re-do your loadout.
And the given, hi-caps are fairly homo.
That said, I use MAG mid-caps (100rd).
However I still carry one hi-cap as a just-in-caser if they are allowed at that certain game I am attending.
I also carry 2 pistol mag speed-loaders (90rd) and have a dump pouch for my discarded mags ($10 from ebay).
I also carry a small bag of bbs in a zip-lock bag so they don't rattle.
Perfect median, and they make playing much more realistic and exciting.
-Brian