Here's my take having participated in one of each contest this year (and also having attended the Nats in Muncie, ID as support crew for a guys competing in Advanced class):

Pattern has a slightly more "serious" setting to it. Contrary to what many say, I didn't find pattern guys stand-offish at all. They'll actually dive in to help any contestant having trouble with their gear. They also seem to be aware that they have an image problem and work hard to promote their competition format. I had a Master's class pilot and a Advanced class pilot offer to call for me during a couple of rounds and their running comment and tips helped me win the contest in the Sportsman class (thanks Danny and Anthony).

Pattern flying only seems boring until you actually try it. Once you do, the desire to match what the big boys can do becomes (at least for me) an irresistable challenge, that you want to acheive.

For sure, I still spend part of each flight with my 120 powered laser just "cutting loose" and doing wild free-form stunts, but another part of each flying session is also dedicated to practicing the sequence and learning from the hard core pattern flyers.

IMAC, is much the same, except that the setting is more laid-back and relaxed. The competition is still just as intense, though. What I like about IMAC is the fact that the planes are much prettier in my opinion. And the free style segment flown to music, was the most fun I've had in years flying a model airplane.