iPhoto is great for most slide shows. It's so much easier that I never use iMovie anymore.
However, with iMovie, you can do some really interesting things with the Ken Burns and Picture-in-Picture effects. Also, if you have videos to include with your photos, you may find iMovie's audio controls helpful.
For me, iPhoto is the way to go because I simply don't have the time to make my slide shows "perfect". YMMV.