At the end of the day, there's no massive difference between the two. You'll get out of it what you put into it although it seems with IMAC you might get a little more out of it in the pocket side.

Apart from a bottle of wine (and a trophy), I've never received anything substantial out of pattern, it's all been money out of my pocket. IMAC seemed to get a bit more sponsor support with prizes and goodies handed out.

Find what's most popular in your area and if they're a good bunch of guys then you'll enjoy yourself, be it Pattern or IMAC. Also with IMAC at the higher levels the planes get bigger and more expensive, at least with pattern if you want to fly in the higher classes the equipment just gets more expensive but stays the same size