Not that I know a lot about soccer but FIFA seems to be light years ahead of Madden. FIFA has better and more in depth features and game modes. Graphic wise Madden just caught up with player models, but animations in FIFA are so much better. So FIFA is definitely much more solid than Madden but I don't know how much modders do gameplay wise. I've used lots of mods on PC games and they are usually just graphical. The graphics on every PC game get improved and optimized with mods. On Skyrim they change the UI and menus. Can't think of any that change gameplay.