Places I'd eat again:

In Buenos Aires I liked Estillo Campo. It's a chain, with one right down from Cabana Las Lilas in Puerto Madero. But a good introduction to beef in Argentina nonetheless.

In Montevideo one must go to the Puerto Viejo. Absolutely fantastic grouping of parillas. Best steak and overall food I had on my entire trip. I made three trips here for lunch, it was uniformly excellent.