I don't like MG because you don't have the sort of control over feeding schedule that you would have with a more bare-bones soil. Also the peat content can be problematic for many growers, especially if you are keeping the plants in the same pot for more than a couple months.
Whatever you choose for your seedlings should be light, airy, and contain no added ferts. If all you can get is pre-ferted stuff, the MG organic isn't THAT bad if you add some extra perlite and flush the soil out before putting your beans in there.