Lappy, the main difference is the IMCA uses a spec motor and a claim system. they also have different tire rules. Generaly speaking the UMP rules are closer to the Xtreme and Boundless rules than the IMCA. They use body rules that are very close and to look at them thier is almost no difference in the sheet metal.
This is a very basic explanation, if you need a better one let me know.
i have been lookin to get getting a LM and they run UMP in ky and i found some IMCA in there area for sale, and was wondering if i could run them in both classes, like a ALL STAR SPRINT and a WoO.
The best advice I can give you in this area is to decide where you plan to race and get a rule book for that track. Make sure you know what you can use before you buy a car.