Whichever election comes first, but I would still think the governorship is his better choice.
I think the election for governor is first. For Colorado, yes, Tancredo as governor is better. For the U.S. as a whole, though, I'd like to see him in the Senate.
More than that, I'd like to see Ken "I'm-a Gonna Put On My Cowboy Hat and Pretend I'm a Conservative Democrat" Salazar booted.