Which is a better time to visit? May or September?
 

Which is a better time to visit? May-June or September-October? I would like to visit during the "shoulder" season when it is not too crowded.

I am planning a trip mainly to Argentina and Chile but might probably need to stopover in Sao Paolo.

Which part? north/south or only Buenos Aires?. What about Chile?. I think for Argentina the weather in April is great, not too cold, not too hot and still long days, while end of September has been so so for me in the last couple of years (mid 50 in the morning/nights). I travel to Argentina 3 or 4 times per year, usually in December/January, March/April and sometimes June (but I won't go anymore), and the end of September and November. I like April or November, but some destinations (like Patagonia) you need to go in December~February, otherwise some activities are not available.

Buenos Aires in October/November is very nice, not cold and not super hot.

I'd agree with this although May can be nice as well. I always recommend these times of year to family and friends. Summer is best avoided at all costs unless you can handle the heat.