Haven't heard really. :roll: Most of the info I get on fish is from wetwebmedia.com and I researched the copperband a lot before since I had considered getting one.
A Yellow Tang will get too aggressive if it's kept in a tank under 75 gallons. It might get aggressive to the point that it would kill other fish.
Concerning the Copperband Butterfly, in general they're pretty reef safe. They don't bother coral or invertebrates but will eat some feather dusters and aiptasia anenomes. The only thing I have against them is that they can be very delicate and difficult to feed so you're going to need to be careful with it. Also, when you feed it, you're going to have to make sure the food is small enough to fit in its mouth because it's mouth isn't very big. One last thing, if you get the Copperband Butterfly, make sure it's eating in the store first (actually see it eat) and also ask that they scoop the fish using a container or something instead of the net because the net could damage their mouths.
So then after the cycle I would recommend the tang over the butterfly because the butterfly will need the tank to have been cycled for a while to survive. It's best to add it after the tank is a couple of years old.