Becky, I've seen metal, metal w/tape, and plastic corner beads. I think a lot of the 'pros' are switching to plastic, because it rebounds better than the metal - once you dent the metal, you really can't repair the spot. Metal can also rust, and is much more difficult to bend around a curve (like an archway). I've seen a plastic product that's pre-cut specifically for going around curves, and does a really good job of it.
I hope this helps answer your question. You can also find some information at this link, under "Drywall forum:" www.jlconline.com