After a cursory use of final cut pro (I use final cut express and imovie a lot), here is what I have found. Final Cut Pro is just a bulked up version of Final Cut Express. Tools that pros would use. It feels like the difference between Adobe Photoshop and Image Ready. A lot of the extras a normal user would never even need. However, I can clearly tell you the difference between final cut express and imovie. When I need multiple tracks, video and audio, i use final cut express. My favorite thing to use imove for is to put together picture presentations, ie the ken burns effect.
Either way, the biggest thing for each is going to be the learning curve and the fact that the final cut programs render later instead of on the fly like imovie (which slows down composition i think).
HA, and cost, unless you are a pro, go with final cut express if you are hell bent on spending money.