We have a vodafone dongle on monthly contract,  but it can be purchased outright, and then topped up with pay as you go.
We got ours from the vodafone shop on the roundabout near hyper simply. If you see the guy in there, Alberto, he speaks very good English. He will advise you of all your options... What i would say is that they are quite costly to run on pay as go, if you are intending downloading music, etc, but not bad for the odd email and browse. Plenty of WIFI cafes now available, for downloads tho, The cafe in Hyper Simply is a WIFI zone.
From experience, vodafone has a better reception than movistar.
To use an English Dongle over here, is costly, and to use a spanish one in the uk, cost approx £20 a day.. you only make that mistake once tho.. Hope this helps,