Neither I'd think. They'd be electrified. But whether EMUs, locomotives, single or double-decker, or length ... it's not been decided.
It will be interesting to see at what point they decide on that. That will likely have a pretty big influence on things like station design (for the infill stations). Hopefully the equipment used for SmartTrack will be the same as for GO RER, even if the differentiation between the two is a different colour scheme.