Is it a suspended floor? For a while now I've mostly been using scandinavian ply for suspended flooring.

For any floor that has the potential to get wet then I'd use ply T&G boards. For any floor that you may want to susequently, traditionally nail hardwood flooring to then I'd use ply T&G boards, ( ply holds the nails better than chipboard.). For carpeted rooms then you could use chipboard. Ply T&G is more expensive than chipboard, roughly twice, but IMO far superior especially in the instances I've mentioned, doing the whole lot in ply may also give you a better price for a bigger quantity. I'd also tie the joists when resurfaceing, whatever material you go for.