I prefer metal. There are so many different types out now too. I have the plastic resonators on a couple of my horns though, and they sound good too. Really, I don't think it makes that much difference.
Sorry to post so late. A year ago I had metal resonators and kangaroo pads put in my Mark Vi tenor. I had to special order the pads. I love them more than I can say. It cost more but it is worth the money and wait.......Kelsey