Sheetrock for fire protection. Also once taped it will act as a better barrier for bugs and wind. To save money you can have them "fire tape" the sheetrock rather than fully finishing it to a level 3 or 4.
Once the sheetrock is up you can selectively place plywood or 2x lumber horizontally against the studs where you need backing for attaching things.
IMO plywood is a much better material than orientated strand board which has poor holding power for screws, nails, cleats for shelving etc. OSB also falls apart when wet.