i spent 5 years of my life in são paulo, and my feelings are of love and hate regarding sampa. If you are planning to enjoy nightlife and bars, i would suggest to stay in Vila Madalena. Around avenida Paulista, Itaim or vila Olimpia are also good options.
An other good "feira" is Praça benedito calixto on saturdays. If you go, a must is to eat feijoada (brasilian national dish with rice and beans) at consulado mineiro.
I wouldn't eat that much meat, specially after buenos aires, but try picanha, a tasty and different cut from the argentine.
Explore liberdade neighbourhood as someone said, and also Bexiga the italian one. I can make you a list of the best italian cantinas.
security: you will listen many horror stories, just don't panic and be more careful than in BA. Do not walk around with huge amounts of money, passport, expensive cameras, etc.
that's for now, feel free to PM me
gábor