You said FCP and Motion and some other apps, which shold utilize those extra cores. The MP is also based on Nehalem (i7 Core) w/ all the benefits of that architecture, whereas, the new iMac has yet to incorporate it. Base on those specs alone and w/ a bit more "future" built-in, the MP wins it for me in this regards. i don't think it would even be a contest. The MP just seems more agile, flexible, durable and darn more powerful. The iMac, to even have a chance would have to get a "lucky" punch in there somewhere in the first couple of rounds. otherwise, the clear champ, in my opinion, is the MP and I think a lot of people would agree with this. it's called a no-brainer.
Of course, this doesn't take into consideration the budget. In addition to the MP, you'd need a good monitor too to use all that horsepower.