It really depends on where you live and the nearest chapter of either club that is close to you, and if it is a group of people that you are willing to spend time with. Relations between the two clubs seem to be fairly cordial, and events are usually not exclusive to club members only, as the events tend to be pay-as-you-go (just like owning an Italian vehicle) rather than provided by the club for its members. There always seem to be a few driving events, a few social events, and a big car show, like Concorso Italiano in Monterey CA, that they organize to attend as a group.
Just for the record, I belong to MCI. I live in Vancouver BC and MCI is based in Seattle, so it's good for me. Since I own a old Maserati, the 10% off of parts from MIE comes in handy.
So, just get on the phone, call up the local reps, and see if you like them! By and large club members are great people, and love to talk about cars and help others out too.
Good luck, have fun!