Telcel does have generally better coverage, including much more 3G (mostly on 850). Those Movistar tariffs do have a lot of small print that would catch out even those who speak Spanish!
There are many dealers and company stores selling Telcel sims (price varies by area, maybe 200 pesos) with a small amount of starter airtime. Easy to recharge at the cash desk of most chain convenience stores in Mexico.
There are a choice of bundles of data (different lengths, different amounts of data) at
Modems USB y Wi Fi de Telcel (use Google Translate, or if you browse with Chrome it offers you translation). You activate them with a text message from your phone.
So it'll probably be a couple of hundred pesos for the card, and something like 249 pesos for Alto15 (15 days, 1.5GB) - there are tariffs with less data but you don't save much . Plus call costs.
If you need the phone for calls but very little data there is currently a promotion where 200MB data is added when you recharge with 200 pesos, but I don't know how long that'll last.