Personally I think .uk is far better, of course I am biased but the numbers speak for themselves.
In country codes we have the second highest saturation of the market in the world and everyone in the country knows that .co.uk is our extension.
Put next to .us there is no comparison, especially as Americans as a whole do not know theirs is even there and while traditionally Americans pay more for domains that us English, they are a far tougher sell when it isn't a .com
What criteria are you hoping to base this comparison on, end sales? average price? ease of selling? market saturation?