Just make sure the paperwork is indeed arranged BEFORE you travel, otherwise, best case scenario you can stay for 90 days as a tourist and then you have to leave, or become an illegal immigrant. Worst case?, you're not allowed in upon landing. Our restrictions in Europe -for legal stays that is- are tougher than yours in the US :))
As per which phone contract, that I'm afraid is still not looking good for someone willing to use the same SIM across several countries. The European Commission is pushing very hard the telecom operators to harmonize their services for European use rather than national use as it's now. However, the directive won't enter info effect until 2017 or 2018 (I believe), thus at present, no company offers a "convenient offer" for a SIM that can be used at reasonable rates in several European countries. If you get Spanish SIM card, when travelling say to France you'll be roaming, thus calls will be as expensive as if you were calling abroad from Spain itself. There are some companies that supposedly offer a SIM card 'that you don't have to change when crossing borders'... but the cost of the calls is far more expensive than if you get a SIM card for each country you visit -if you're going to stay several days that is, otherwise it's not worth the hassle.
Having said all that... assuming that, like me, when travelling abroad your phone is simply to get calls in case of emergency, then you don't need any further SIM card, just use the one you buy in Spain (but avoid using it for chit chat or for browsing the Internet!)