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.