The difference is minimal. MFK used to be a site of "big fish, big tanks whose owners spared no expense..." But things have changed, and those words have been removed from MFK's welcome page. Most of the members on MFK now define "monster" as having passion about fish keeping. With that definition of "monster", Since members of both groups both have small fish/small tanks in general, the difference is that MFK members have more passion than AC members
I joined MFK in 2006 because I had questions about filtration for a 300 gallon tank. In 2006, big tanks were the norm on MFK. It was the only forum that had discussions on set up and filtration for larger tanks. I probably don't meet the requirements for being an MFK member anymore, because I don't have enough passion about fish keeping. I have, however, graduated to bigger tanks and have a 405 gallon tank to use for redundant sump filtration.