Hi Mike,
I think it's important to keep in mind just how big Argentina is and how many Koppen Zones Argentina has (11 according to a quick search, 2 more than the lower 48, though different of course; we don't have nice Florida like beaches sadly).
I personally love Buenos Aires from May through October, New England Fall is my favorite time of year (truly missing it right now) so for me it's perfect, but if you find it too cold you'll have to look north with the exception of high elevations in the Andes and the desert in Gran Chaco which gets pretty chilly at night, I assume like Arizona in the winter.
The most tropical part of Argentina is Misiones in general, and Iguazu specifically I believe. The median temp is 16ºC to 22ºC from May through October, though October is the wettest time of the year in Iguazu.
A better option would be the Corrientes/Resistencia/Formosa area, the median temp is 15ºC/16ºC to 22ºC/23ºC from May through October like Iguazu, but much drier. That being said, aside from the cities, Chaco and Formosa are the poorest provinces in terms of HDI, so cheaper than BsAs but more poverty/less opportunity, and Formosa specifically is often called a dictatorship as the same man has governed it for 30 years.
