Its actually a very good question. There are two sections on the main massage websites, masajes.com.ar and todomasajes.com.ar, and they are "masajes terapeuticas" and "masajes sensuales". I am a big massage guy so I will tell you what that is all about.
Masajes Terapeuticas: Basically this means that the girls are "masajistas matriculadas", or trained massage therapists (in theory anyways), and it means that they have studied massage and they should give you a real massage. They are in general not as good as professional masseuses back home, but some are actually very good at what they do, and they expect to give you a massage for about 45 minutes if you book an hour session. In almost all cases unless otherwise mentioned with something like "NO SE BRINDA ATENCION SEXUAL, NI GENITAL, DE NINGUN TIPO NI CLASE. POR FAVOR, NO INSISTIR" they will finish you off with a happy ending. These happy endings vary in quality and sometimes these girls charge extra for this, but the massage (in theory) should be good. Prices vary, but a standard rate is 80 pesos hj usually included, extra for BJ and sex. Very few of these girls will do BBBJ, CIM, those kind of nasty things, it is basically what separates a massage girl from an escort. Some will not do sex, no matter what you offer. The line isn't so clear but basically in their ads they aren't naked and they don't consider themselves prostitutes.
Masajes sensuales: These girls are usually better looking and their service is implicity sexual. Most of them provide sex, but that doesnt mean its included. Prices vary, and services vary so it behooves you to speak Spanish and find out exactly what is included in the price. For example, this girl...
http://www.masajes.com.ar/vip18/index.html
Asked for 150 pesos, but I negotiated her down to 120 and to come to my house and completo was included. She actually asked me what costume I wanted her to wear (I chose school girl ) and we started making out right after she arrived. She gave me a massage but you could barely call it that, and she was basically an escort. Actually she had the most beautiful pussy and ass I have ever seen in my whole life, but that is a story for another post.
Some of the masajistas sensuales advertise as escorts as well, and don't even have a massage table in their place. If you are looking for a good massage your better off looking in the masajes terapeuticas section, though some of these girls are quite good as well. Just like any mongering activity your results may vary, and its good to read the local forum and see which girls are recommended. They tend to rate girls on many factors, including their massage ability, but many Argentine mongers go to massage girls just for sex. These girls tend to advertise naked and they are openly providing a sexual service, I guess more than anything that is the difference between the two types of massage girls. Rates vary from about 60 pesos/hr to 150 pesos/hr, and it is good to find out just what that includes.