If you want to go on international form alone, Spain clearly have been the form team on the planet since EURO 08. Brazil have also been good since Copa America also, but they aren't a settle team IMO.
If we jude them player vs player in a potential Best XI:
Casillas>>Julio Cesar - Casillas is argubaly along with Buffon the best keeper ITW. Plus when you have Reina as back-up you pretty safe.
Ramos>>Maicon - Both are equal going forward. But Ramos is better defensively i feel. But if its Ramos vs Dany Alves, its dead even.
Capedevilla>>>>>any potential left-back they could chose. We looking the likes of Marcelo, Maxwell, Santos that played in in the Confed Cup, Adriano from Sevilla or Tottenham outcast Gilberto.
Puyol & Pique>>Lucio & Alex/Juan. Pretty close in a potential first team. But spain have Marchena, Albiol, Jaunito as back-up. Compared to Naldo, Luisao, Thiago SIlva. So Spain depth is better.
Mid-field although Brazil have Kaka & Ronaldinho. Spain clearly have more depth here & have a settled first mid-field quartet once fit of Senna/Xavi/Iniesta/Silva. While Dunga still isn't sure how/if to accomodate his two most creative mid-fielders.
Villa & Torres forms the post potent strike force in the world ATM. Fabiano & Robinho are top, but a tad behind.
If they where to play i'd back Spain to win, because they could would dominate the mid-field battle.