Hi marci. Well, it depends on the amount of time you will be here for. If it is under 6 weeks, it would be better to rent a phone but for longer stays, just buy one.
I seem to be just about the only one who rents cellphones in Spain as far as I can tell. I did EXTENSIVE research before investing in a load of phones and if you go do a www.google.com
search with "cellphone rentals spain" (or any combo thereof) as the search terms you will see my service come up first (don't ask me how I got that high up).
If you look at what is offered out there, you will see that renting a cellphone in the U.S.A. prior to departure is NUTS! The calling rates will have you sleeping in the streets and eating...well...NOTHING during your entire trip!!! Most U.S. companies are way out of control on this topic.
You can learn more at www.onspanishtime.com/web/cellphones.htm