MacBook pro vs. iMac for fcp?

I heard Final Cut Pro X is coming out and I was thinking of swigging to Mac. I am wondering whether to buy an iMac (hopefully a new version if released soon) or a 15' MacBook pro. What are the differences in terms of speed of editing? I can't stand when the playback lags or it is slow, so I'm wondering if I will be happy with a MacBook pro. How much faster is the iMac? What is the life compared to a leading pc in terms of speed and performance? Any information is greatly appreciated!