I don't have any 290/390 but I have 280x and i'm currently mining ZEC. I think you should too
Whats your hashrate on it? and the power consumption?
290 stock bios (elpida): 275h/s zec
290 stock bios (hynix): 290h/s zec, 27mh/s eth
290 stock bios, core oc (hynix): 29mh/s eth
290 w/390 bios (hynix): 330h/s zec
290x is almost the same h/s for zec, slightly faster for eth
only buy 290s if you can get them extremely cheap, or have free power. otherwise, 470s are better for eth, and nvidia are better for zec
Do you happen to measure the power consumption?