Brainfarts, rashness, petulance, inconsistency and inability to stay fit. Valencia is the opposite so its fairly obvious why he gets the nod. If we want to ever get to the stage where we can field a consistant back 4 week after week then Valencia is the one of the 2 we need to persevere with.
Out of those descriptions, the only ones I'd agree with are "inability to stay fit" and "inconsistency" (in a way). He's been less rash than many full backs that others rate, and the "brainfarts" and "petulance" was something he showed before he matured. When he's played regularly, he's been consistently great. However, when injuries occur, he becomes more inconsistent, which is understandable given that he never gets going.
Yes, Valencia is reliable in that he follows instructions to the point and you know what he brings to the table every match. However, he doesn't have the quality in his game that Rafael has. Valencia is very reliant on his physical traits when defending, unlike Rafael, who's very good at marking players and positioning himself, defensively. Going forward, Valencia has improved under van Gaal, but he's still very ponderous, unlike Rafael, who's quicker on the ball and has possesses more variety and quality in his attacking game.