What to use...

[Dr Stan]

I'd go with the 1018, just use some anti-seize compound during assembly. The idea that SS is stronger is somewhat misleading. It is tougher and that's why it can be difficult to machine, it also springs back from torquing better than 1018 which is one of the reasons you'll find it and monel frequently used in pump shafts.

IMHO save the SS for a more appropriate application.