Over the years I was lucky enough to keep both at times. Selection of both of these is probably more important with these guys.
If you get a Copperband, make sure it's eating at the LFS, really eating, not just biting and spitting out. Some just don't and die. Most of the ones I've kept will go crazy for live blackworms, so that may be all they eat to start.
Mandarins I've had never ate frozen, so there needs to be plenty of pods to sustain it. Only one of the few I tried ever survived. If I remember correctly, he had a nice plump belly at the LFS. The others I tried that died after just a few months, had pinched bellies at the LFS and no matter how much they appeared to eat of the rocks, they never fattened up. So, I'd recommend looking for one with a nice belly, not sucked in.
Good luck with whichever you choose, they are both beautiful fish.