Hello Matthew,
Traditional at large sailing vessels the bobstay was made of a chain, my WB is fitted with a 6 mm stainless steel bobstay, I would recommend this strongly.
Chain material should be 1.4571 (because of corrosion resistance) or if you prefer areas with saltwater 1.4462.
Unfortunatly it is difficult to get fittings, turnbuckles a.s.o. made of high grade stainless steel. I have chosen AISI316 because it is easier to acquire.
At least I think, if a system is as strong as its weakest element AISI 316 (1.4401) will be the practical choice for chain and elements.
Kind regards
Gerold