If you're talking about coverage in Buenos Aires, there's not really anything to choose between them. A few years back I would have warned anyone off CTI (now Claro) as you used to lose the signal everytime you entered a building - the amount of money I spent picking up voicemail messages and then calling people back!! As far as the rest of the country is concerned, I now use Movistar and the service has been fine in all of the touristy areas I've been to.