alot of people are having issues with nickel on the subtanks. I have had my share of blues with them too, even on the xcube 2
For those who can temp control SS, I'd say use SS. Works great and no worries of oxidation.
For those who CANT temp control SS, I'd still recommend SS. Works wonders in Wattage mode, and no worries of oxidation.
Correct me if I am wrong tho ^^