FCE capturing
For basic capturing file sizes and quality will be pretty similar, as they both use standard DV. The biggest difference in sizes will come in more advanced editing.
For example if you wanted to play the same clip twice, or you parts of certain clips multiple times. In iMovie it would duplicate the file, doubling the size. FCE will just use software pointers to the existing video files.
This means FCE uses space more efficently, but also plays alot harder. You will need a much more powerful hard drive to get good performance.
For example, my internal powerbook hard drive runs fine with iMovie. However FCE is next to unusable with it. My 7200 RPM external firewire drive is need to use FCE.
However iMovie 3 is so slow, buggy, nasty, annoying, and evil, that I would suggest that everyone who actually wants to do some neat video stuff buy final cut. It is an amazing program, and it's posibilities are next to infinite.
btw if anyone can explain the FCE audio rendering thingy, please tell. It is one of the very annoying parts of FCE. Thank gosh rendering it is so fast.