Tough call. I'd probably prefer to see him take on Gregoire. WA gun laws aren't bad, but they could be better. I suspect he'd be better able to enact legislative change at the state level than over in D.C. where one senator's vote only goes so far.
I've been given the impression that he's more interested in the governor's mansion that the US Senate, but I could be mistaken. I'd vote for him in either race.