Final Cut Pro has some more fine tuning sorta features on just about every effect unlike final cut express. it runs much smoother with various video formats and is very easy to use with all of the other programs that come in the FCP suite.
If your looking for something better than iMovie but not looking to make a lot of edits or get really serious about learning a bunch of new software, get Final Cut Express
If your looking to really get into editing and willing to learn the full capabilities of all the software and the cool stuff you can make with it all, get Final Cut Pro, you will not regret it