have a look on the Buenos Aires zoo website -- last year in the summer they had tours to the zoo at night -- started about 9pm. Zoos in general make me sad, but I don't think that the BA Zoo can be all that bad - they have had a lot of success breeding big cats there (and I believe have a program with Temaiken for the breeding). The enclosures are not very big though -- I haven't been for a long time so not sure how things are looking now.
Temaiken is a very nice setting, and preferable to the BA Zoo, but remember with the heat that if you don't get there first thing in the morning you're not going to see much. Zoos aren't like gameparks unfortunately -- they don't open at the crack of dawn, so within a few minutes, maybe an hour of opening time the animals are likely already passed out!