After seeing the Space Gray model in the store, I think I'm going to get the Silver model. Space Gray looks to drab to me. I've always liked the look of the Silver MacBook Pro.
I'd go for space gray. Everybody has silver macbooks around me, and it would match my iPhone 6 Plus. However, I saw on a review that the Apple logo doesn't light up on the space grey version, so I'm a little divided on that. Depends on how much you love the glowing Apple logo, I guess.