Dark, saves energy with amoled display. Also better for life expectancy. And use a green accent color, since the green leds have the best life expectancy.
I used to setting dark, but after 8.1 I wanted to try out white with some backgrounds and white themed is just awesome with my white 920. I really can't imagine now going back to dark so can't even think of downgrading to WP8. My brother's 720 is also set to white now and you can tell he also enjoys it.
80% of the time I use dark, 20% white... Depending on the color that I use because some colors looks better with dark and vice versa. And the dack theme in the night make everything seems like it's floating *_*