I tend to root for the best team (of players, not necessarily the best coached squad as I think Brazil is playing better than Argentina for the most part but with a slightly-less-talented squad) regardless of nationality. The last couple of cups that has been Argentina. A lot of times that is Brazil though, yes but this year they have a nice disciplined team but hardly the best group of talent in the cup (I'd put Argentina and Spain ahead of them on paper, but in Spain's case you have to automatically factor in World Cup shrinkage).
Messi has the skill to go down as the greatest player of all time when he's done, he's that good and that young (isn't he 22?!), and I root for the talent to shine (despite their coach etc).