I don't think there's much of a difference. Most homes in my area use metal. And wood registers are becoming more popular with people who have hardwood floors. Just make sure the ducts and registers are the right size, that's more important than what the vent gills are made of. Some prefer one over the other, but I don't see much of a difference.
Are plastic or metal registers the best? Does it matter?
I bought plastic initially and have not been happy with them. They just have not held up very well at all and I have started replacing them with metal. Of course, its possible that I just got a bad batch of plastic registers.