Raw DV is Raw DV, and the format is the same whether you capture in iMovie, FCE, FCP, or Windows XP Movie Maker.
(Well, the raw DV will be wrapped differently -- it could be wrapped in a QuickTime or AVI wrapper, and there's technically Type 1 and Type 2 DV files, but all of them are nitpicky things that have rather negligible impact on file size.)
Either way, the transfer rate is about 3.5 megs per second of DV video, which translates to roughly 13 gigs of hard drive space per hour of DV video. Remember that when editing DV you'll need at least two or three times that much space to make room for scratch files and rendering.
The key differences between iMovie and FCE? iMovie was designed from the ground up to be easy, easy, easy. FCE is not. FCE will have far more functionality insofar as non-destructive clip editing, adding effects and layers, compositing, color-correcting, and so on. iMovie does a lot (impressively so) but there will be times you have an idea in your head that iMovie simply won't let you do. Chances are, there's a way to do it in FCE. FCE, is, in turn, a stripped down version of its bigger brother, Final Cut Pro.
I guess it's like driving stick versus automatic transmission. With the automatic you get in and go. With stick you train a little bit longer, need to get the feel for things a little bit, there's a learning curve -- but it's not THAT different. For most people, the automatic would do them just fine. But for those who need "more" -- whether it's power, flexibility, special tricks, or just the feeling that you're more in control -- there's a lot you can do with a stick shift that you can't do with an automatic.
That said, I drive an automatic
