I've always maintained that Barca made a mistake letting Thiago go (it was sort of poor management) but the current hype around Thiago is insane (not on this forum necessarily). This is the same guy who couldn't displace Cesc from central midfield and was making big mistakes every other game.
Rakitic easily, and I'm not even a huge fan of his.
In the system we're playing definitely Rakitic, just because of the defensive cover and workrate he provides, but ability-wise it's not even close. Thiago looks set to be one of the best midfielders of his generation, while Rakitic is a good player.