Experiences will vary as to which one is the better grazer. For me, the foxface is pretty much a freeloader! My yellow tang (as well as all the others I've kept in other tanks) is a tremendous grazer.
I should say though that I have much experience with yellow tangs, and comparitively little with the foxface. The foxface is shy, and they take a while to warm up to things and act normaly. For now though, he hangs out and waits until I feed the tank for the most part.
As far as hardiness goes, no worries either way. In the end, the foxface would be the hardier fish as far as disease resistance goes. Like I said though, it really isn't an issue with the yellow tang either. Just make sure you quarantine.
The foxface is also much less aggressive than the yellow tang, so from that standpoint, he might be a better tankmate for a copperband.
Cheers
Jim