I always choose the outbound via JFK, because it has the shortest total travel duration. Leaving London after 5 PM and arriving in São Paulo the next morning doesn't feel too absurd compared to the non-stop options. Never had a problem clearing immigration with some lounge time to spare. Also, it's the same aircraft continuing, so if you pay for WiFi it's valid throughout
. Returning, the day flight out of GRU via MIA works nicely (although it sometimes connects with an old-configuration 777).