I'm over 6" girth so I've tried both. Never had much of a problem with either. For me the Magnum is a little tight, and the XL actually fits better, but I buy Magnums out of embarrassment. Even though I know that the difference is girth I can't bring myself to use XL because some part of my mind thinks length is the difference (and I don't have big length).
Despite my own paranoia and insecurities, my advice for condoms is always to go with what feels the most comfortable. If the condom is hard to get on and feels like it's pinching in on the base, move up a size. At the end of the day, it's not about ego, it's about protecting yourself from STD's or getting your partner pregnant.