It's interesting to see age playing such a big part in colour choice. I'm 28 and my previous A3 was metallic dark grey, this time went for Glacier White as I felt it worked better on the car. Next time I might go for black, with the black grill / styling pack as I think that's quite a distinctive look too.
With regards cleaning, my friend's phantom black car is actually harder to keep clean than my GW, as it shows every single bit of road dust and takes forever to clean properly in order to avoid swirling the clearcoat. My GW on the other hand doesn't show any dust, but does show mud etc, but at this time of year that's not such a problem. I was unsure about GW when I ordered, until i saw it and then i knew it was the right choice for me :)