And By the way, to answer your real question: FCE HD does not crash as frequently as iMovie HD for me, very rarely does either one crash though so I'm not sure that's a fair statement.
Save early, save often.
Unless you're a serious hobbiest or higher, I wouldn't go to Final Cut Express just to get a more reliable environment though. The learning curve is steeeeep. Don't get me wrong, if you're into it, it is a blast, but just be aware of the learning curve. For example what you may be doing in "10 minutes" in iMovie, might take you 4-6 hours to do in Final Cut when doing it the first time, just from all the basic interface knowledge that is necessary.