are you kidding me? a white car shows dirt WAY more than a black car... but back to your question...
scratches will probably be harder to see on the white I'd say, but i have a black and I've gotta say, unless your scratches are REALLY bad, they aren't that visible. I have a few small scratches, but you can't see them unless you really look for them in the correct lighting.
anyways, the back of your iphone will scratch no matter which color you have, unless you put an invisible shield on it or something (i know people who've gotten scratches even with cases on)
also, the white will probably show dirt more, but the black really shows fingerprints a lot