in my mind it's no contest.
the iphone 6 seems like it was designed from the ground up for the black/space grey version. even all the promo materials for it show it in black.
this is in contrast to the 5 which seems to have been designed for the silver/white.
The curved screen makes the phone sleek, a departure from the simplicity of the 5's design language which matches well with the minimalist white colour. further, the curved glass causes weird shadowing on the white bezels on the 6 - not evident on the black.
add to the fact that the 6's meant to be seamless, the black camera and proximity sensors on the white face sticks out way too much and breaks the whole seamlessness of the phone. on the black, the whole phone seems seamless - even the screen seems to blend into the bezels itself - especially evident with the pink flower wallpaper, shown on a lot of the promo materials.
you can't get that effect on the white iphone - so you miss out on that design 'feature'