Forget those mag carriers with the plastic retainer clip, by the time you undo the plastic clip to reach for your extra magazine - the BG has probably shot / stabbed you to pieces or both.
Try retention holders like:
https://secure.fobusholster.com/cata...x.php?cPath=28
For about 15 bucks they are a bargain, and with practice, you can reload in under 3 seconds. Plus they are paddle mounted - easy to remove and put back on without taking off your belt.
I have found - these are most convenient way to carry extra mags - unless you prefer pocket carry.