I asked a lot of people in BsAs what cellular carrier was best, and virtually everyone said to stay away from Claro (bad signal) and results were split between Movistar and Personal. A few things I learned about them - With Movistar, if you load 20pesos of credit, they add another 25pesos, total 45 pesos.

However, Personal does this too but only on certain days of the month. I ended up choosing Personal, but this is because many of the people I know in BsAs have Personal, and its half-price to speak to them.