Depends on what you want to do with it. Generally speaking both are powerful enough. That being said, if you dont plan on every having to take it anywhere, I would say definitely go with iMac. For $2000 you get a quad core processor (compared to the duel cores in the MBPs. While MBPs do have core i7/i5 processors, the mobile versions of them are only duel core), much more powerful graphics card, higher resolution display, and generally higher end components.
That being said, i just got a 15" Macbook Pro, the Core i7 model, and i love it. I have done a small amount of messing around with final cut and it runs great. Plus it has the added convenience of a laptop. My reasoning was that it will be my college computer and I will want to be able to take it around with me.
Basically, either one will be great, it just depends on your specific needs.