Thought I'd chip in as a carioca: Sao Paulo definitely worth a couple of days, you can visit the Central Market, Football Museum, but most of all the food and nightlife scene which put Rio to shame.
Nightlife in Rio is dreadful. And by this I mean nightlife in the sense where you would refer to in the great nightlife cities of the world like New York, London, Miami, Berlin, Milan etc. sure, you can put on some flip flops, go to a run down bar in Lapa, drink appalling Brazilian beer and listen to some samba, but if you are after unique venues, sophistication and world class electronic music, Sao Paulo is a great place to be.
My only caveat would be: SP is much better during the week, as locals leave town during weekends. However, weekends are when you'll find the cheapest hotel rates (Jardins is a good place to be based). Conversely, Rio nightlife only exists between Thu-Sat nights.