WMM = Crap
The difference between iMovie and WMM imo is that iMovie has more fancy transitions, and a nicer looking UI. If you ask me I hate both of those programs. It's too simple. I've tried using both and compared to Adobe Premiere CS4+, they suck.
If you've used Garage Band before and you are familiar with the overlapping of different layers and timing of music you should be able to use Adobe Premiere easily. You can edit your video more freely, as there is a distinction between the video/audio layers and even add cool animation effects with Adobe After Effects (although I haven't used this properly yet). What WMM/iMovie offers is basic, and depending on what you're editing I'd recommend using Adobe Premiere.
I've heard Sony Vegas is good, but I struggled trying to install it so I don't know what their software is like.