I would recommend you stay in the hotel with the best value or meets your preferences. Buenos Aires taxis are very cheap. For example, we stayed at the Marriot San Martin which was a grande dame of a hotel and was $3 US to the major bars/clubs.
I would second the Puerto Madero section as the hotels are newer. The Hilton has been highly recommended.
The Alvear Palace was also very regal and elegant. Excellent dinner and we were treated very graciously.