White is awesome.
My wife has had the white iPhone 4 for months of heavy use -no case- and there's not a hint of wear showing. If you're not an exceptionally filthy person, the finish is such that it doesn't collect grime. You can also put a transparent guard over it if you're really worried about that.
I find the white to be exactly the opposite- black finishes tend to show every minute scratch and flaw (far more likely than common grime, unless like I said, you're a particularly filthy person) whereas scratches and tiny flaws in the white are practically invisible.