How far is it from Sao Paulo to Rio by bus?

You should definitely go by plane. The flight is about an hour and a half long. Although it will be more expensive, it's not worth it going by bus. The actual distance is 425km, and the road between the two cities, the Dutra, is almost constantly packed with traffic. I'd say it'd take you at least some six hours by bus. So I'd go by plane is I were you.