OSB should be cheaper by a few bucks. Nothing wrong with OSB if it's thick enough for the application. Is this something you will walk on? I think minimum thickness on 16 centers is 5/8 for floors regardless if Ply or OSB. Might even be more by code now. Not to say you need to do that for a storage area in the rafters. I don't have any floor in mine. It's open from the garage going in above the house area. I just walk on the trusses if I have to get in there. (wiring, etc) Haven't stuck my foot through the sheetrock .. yet.
