XAV-
I can speak very clearly about the answer to your question. I have owned a pearl white Model S, and now I own a silver Model S. So I've lived with both, and I'm clean car freak. The actual reason we went with Silver Metallic on our second Tesla was specifically because the pearl white was a bit hard to keep clean. Something that no one has addressed here in their replies is the fact that because the Tesla has electric motors, it is quite literally a dust magnet. Think of how your old tv would attract dust. Any particles out there that are negatively charged will actually attach themselves to your car. Wash your car, go out for the day, come back and your Tesla has a layer of dust that is has 'magnetically' attracted to itself.The white car really showed this dust. All Teslas will do this, but we found the silver metallic to be good at hiding it. Sorry for the long explanation, but the answer to your question, from first hand experience is :Silver is a lot easier to keep clean than white.....on a Tesla.
Now if only Tesla would offer Carbon Ceramic Brakes like on our 911 Turbo. NO BRAKE DUST EVER! I'd pay anything for carbon ceramic brakes from Tesla. I hope the next roadster has them. They're lighter too, so why not?