I think the Phillies see Rojas as having more power potential, but Garcia having a bit more speed and defense.
But as with any set of young prospects, it is better to have two than one. I also like that both are seen as potentially plus defensively. Maybe the organization will take defense more seriously as some of our "hit tool first" guys like Bohm and Stott are less good defensively (not that Stott is awful, just that if he sticks at SS it would be because the bat plays well there).