Tariffs have fallen dramatically due to the price war that has been raging between the two service providers in the last few years. Before buying a phone or signing up for a service, compare the prices of the various services offered by all providers.
In Spain, there is the system of top up cards and subscriptions, but with some providers such as Movistar you will find contracts without subscription, mandatory duration or other fees. Payment of your bill can be done by direct debit or by mail with some banks that will be specified to you.