Thanks, #6, for your input. I didn't get to stay in any of the presidential suites, so unfortunately my report isn't quite as glowing. The room was small and didn't have upbeat, redecorated bedding. I moved from a low floor to a higher one because of some strange glue odor (some kind of rennovation work was apparently going on). The higher floor was nicer. The staff was friendly and helpful, with the exception of one person at the front desk. I didn't use the pool or sauna, but I stopped by and thought it looked nice.
You're absolutely right about the location being ideal. The Park Tower is not inconvenient, but I felt more comfortable taking taxis at night when the immediate surroundings became very quiet. The Park Tower obviously costs a lot more than the Libertador, but it is a luxury hotel that may be worth the extra (although I would stay at the Alvear if the rates were similar).
In short, I would suggest the Libertador if you can get a rate under about US$65. Otherwise you can do better in BA for the price; it looks like you can usually get the well-situated Claridge's for under US$100 these days. For a category 1 SPG award I picked up during the 25% discount days, I was satisfied with the Libertador.