Used to live in Argentina. August is an okay time to go as long as you keep your expectations in check. It's cooler and darker which can be tough since it's not uncommon for restaurants to open later for dinner there than what we are used to here. I'm not a huge BA fan. I enjoy Patagonia, though will definitely be cool. If you go up to Iguazu Falls will be a little warmer. If you like to ski, you'll probably be able to catch the end of ski season depending when/where you go. On the plus side, it's not their big tourist season so it will be quieter.