You will find the best overview here:
http://prepaid-data-sim-card.wikia.com/wiki/Spain
There is a new one available, not mentioned there yet, however if you only need data, the mentioned ones seem to be more suitable:
http://movil.orange.es/tarifas/llama.../sim-holidays/
For larger data amounts you could also consider buying two SIM cards to replace the first one as soon as the allowance is used.
The advantage of Vodafone and Orange cards is that normally they are available at almost all Spanish airports at Crystal media shop (watch for it near other shops close to boarding gates before leaving the restricted area).