Well I have both Magpul E-mags, and MAG brand mids. I've never had an issue with either in my weapons. If you don't get a mag picked out and we actually go to the same event I'd be more than happy to let you run one of each and check then out for yourself.
The Echo 1 Dogs of War 100-rd mids are the best I've ever used. I got freezing water/mud/snow in them, and they still fed. Dunked them in a river by accident at RAGE's most recent REC game, and they got filled with sand. Still fed And they're metal.
I have a bunch of old MAG midcaps that I could be persuaded to part with. I'm probably going to be at the Field Day at No Limits. You could try and buy. I'm guessing something like $30 for I think 8 or 10?