the color only provide you with the ablility for you to use it as an asseccory, some people like to stand out and not just settle for black and white they need grey areas.
And the choice of colour really depends on what colour you prefer... Im telling you right now, both colours will be colours for ipods for a long time. Both will scratch although black ones show more i guess (get a case)...
But silver is also too common among TONS of things... Black is less used.