Alot of people here will suggest for you to maybe go with Telcel here because they have more coverage, however, I will tell you that I have traveled to several parts of Mexico and had excellent coverage with Movistar everywhere I am. I travel to Monterrey at least once a month and have had excellent coverage there as well. Where are you specifically traveling to? Also, in regards to pricing, Movistar has Telcel beat hands down, look at this page here: http://www.movistar.com.mx/n1/n1_prep_tipo.html
They have a prepaid plan called "por llamada" which charges a flat rate per call, I use it and its excellent. Anyway, let me know if you have any questions in regards to the cell service there, I;ve tried both Telcel and Movistar and know alot about both.
Another thing I would like to add is that its difficult to find the sims by themself, usually the best plan is to buy the phone and sim, its not much money, usually $35 or $40 or so.