Definitely ETC + Sia better than Zec currently 
Not if you are paying power eth + SIA is power hungery by like 30% , Eth alone uses a bit more power than ZEC alone so its a no brainer to mine ZEC if you have power over .10
If you have the R9 280 or 7970, it is better to mine the ZEC. If you have the RX 470 or 480, it is better to mine the ETH now.