We get more requests for brass than bronze and have never had a request for copper on a knife. I never have understood why people put copper on knives when it tarnishes so much quicker than either brass or bronze. It seems like the people that frequent the forums universally hate brass, but yet knives with brass guards sell very well. Bronze would be my choice since you still get some color and tarnishing would be much less of a problem. It is a little harder to work to me than brass or nickel silver, but I like the looks.
Brass is a very traditional material for guards on swords and knives that has fallen out of favor with many collectors in the last 20 years or so. I like old looking brass myself, that has that mellow patina that can only be gained with age. One of the best looking knives I have ever seen was a Loveless with engraved brass bolsters and a sheep horn handle. I'll bet it has been a while since he made one of those.