He chose Lloyd because Lloyd was the cuter of the two. That's what it comes down to. He had the sweeter face. I think in his head he had this idea of Vader being the most un-Vader-like kid in the world, a sort of poster child for The Good Kid, maybe someone he saw the way he saw his own son, and Lloyd lacked the acting chops but had the right image. The runner-up was more mature looking and had a bit more edge to him (he also looked way more plausible as Mark Hamill's father), but Lloyd had that wide-eyed, rosy-cheeked looked that Lucas thought was more important. Style over substance as usual, I guess.