When choosing a health care company in Argentina the main aspect to consider is in which clinics or hospitals it covers hospitalization or ER: that is where the costs are really expensive if you're not well covered. It should cover hospitalization at least in the Swiss Medical clinic, Mater Dei clinic, Otamendi, Instituto del Diagnostico, Hospital Austral, Fleni, and I'm sure I'm leaving some good ones out.
If the plan you choose has good hospitalization, the specialists that are included are also, most likely, good ones (English speaking is a completely different story).
Try to stick to the major health care companies: OSDE, Omint, Swiss Medical. Hospital Aleman has been mentioned (and recommended) many times in this forum, but I have never been there personally or know their plans.