Well, it's damn near impossible to describe sounds, especially with such subtleties as these. To most ears, there's no real difference in the timbre of an instrument based on the metal. But all the same, the softer the metal used, the more mellow, or "warm" the sound produced. That's why silver is best for symphonic instruments. Brass instruments generally have a more brash tone, and so are preferred for jazz or marching. Another factor in the sound is whether or not it's lacquered or plated. A lot of big band players in particular prefer the sound of brass instruments that have had the lacquer removed.
Plus, I like my silver trombone because it's pretty.

I wish I had the time to actually play it, though.