Spain look stronger on paper
But it's football and on field anything can happen.
Argentina has one huge advantage over Spain, that is experienced and the power to suppress all kinds of pressure..
Most of Spain squad are young star and i believe most of them played WC first time and not to say they idolized Messi, definitely there would be some pressure.
As long as they play free flow game, without succumbed to the pressure like they bullly France, victory is given....i'd say