I would say that it depends. What are you editing? Do you have experience with another editor? For simple projects, I like iMovie 08 just fine. Very fast and easy to use. If I need to be messing with fancier edits (like J-cuts or L-cuts) then iMovie HD is better. But then, if I am working on a project like that, I am likely tempted to jump up to Final Cut Express.
if you have iMovie 08 why not just try them both out and see which you prefer. iMovie HD is a free download now, so it is easy to try making a little movie with some similar qualities to the project you have in mind and see which works best for you.