The so-called Sao Paulo International Airport is in the nearby city of Guarulhos (even announcements in the airport now just say "GRU"), so you wouldn't exactly be "in" Sao Paulo when you land. Lots of flights to Rio, both the larger Tom Jobim Airport (GIG) and the smaller downtown airport Santos Dumont (SDU). Of you could go to downtown Sao Paulo and take a bus from there to Rio. Certainly a lot more to see than Rio and SP. Paraty, Tiradentes, Ouro Preto, etc., etc., etc. Friends love SP for its museums and shopping. Rio is certainly more picturesque, and worth a visit, but I think there's also some hyperbole regarding its charms (I worked there for 2 years and have visited since). Get a guide book and plan ahead. Number of days one "should" spend in each place depends on their interests. Do some research first, then ask questions.