As usual, for antennas, the better your material conducts, the higher your antenna efficiency. Hence, go for copper. It has an excellent conductivity per volume (which might or might not be critical in your application), and there's not many metals that will beat its conductivity at all. Steel really isn't good for that.
If you're using suitable frequencies, steel wire with a copper coating might work for a dipole. The frequency will define your skin depth and the losses that a non-perfectly conducting antenna will incur. Definitely sounds like you'd have to just copper-coat some spring steel and try.
Another thing: I don't know how stiff the deployed antenna will be, but you are in the earth's magnetic field, with long ferromagnetic rods; that will excert a torque on your satellite... I can't calculate how much, though, since I know nothing about your antenna sizes, your orbit, your height or anything about the satellite.
Generally, when asking about "what's the perfect material for my antenna", you should include a lot of factors:
- frequency (which I find kind of obvious)
- acceptable loss
- mechanical forces