I have also researched the SIM cards. I'll be coming in from California and Verizon is very, very expensive for an international plan. My friend who lived in Europe said the Vodaphone has the largest coverage in Spain. I put together some notes that I will share with you. Good luck on your decision and please let us know how it's working out when you get there!
Spain
Spain has four network operators, Movistar, Vodafone, Orange, and Yoigo. As of 2016, Vodafone had 95% 4G coverage, followed by Orange with 89%, and Movistar and Yoigo each cover 86% of the population. Movistar is known to have better coverage overall, but check the areas you specifically plan to visit. Spain has a healthy resellers market, but most don’t offer 4G on prepaid SIM cards.
Your Options
Both Vodafone and Orange offer a tourist SIM card. With Vodafone, you can get 2GB of 4G data and 50 minutes of international calls for about $16, and Orange offers 3GB of 4G data and 30 minutes of international calls for about $37. Movistar charges you about $11 for a SIM card, and you can get another 1gb of 4G data for about $10. Yoigo charges you about $21 for SIM card but you get that amount in credit.
Other Tips
Spain requires you to register your SIM card, so you must bring a photo ID. There are no branded SIM card provider stores in the major airports in Spain and the available SIM card stands are overpriced, so wait to buy a SIM card if possible (there’s free wifi at the Madrid airport). See here for more details on SIM cards in Spain.