This^^^^If he wants to go to the Warriors, he has to go now. They won't have the cap space in a year. If he re-ups in OKC on a 1&1 and then he and Westbrook decide to part ways, Boston is probably the presumptive favorite to sign him next offseason. Incidentally, staying in OKC for one more year and then leaving for Boston is probably the non-Warriors option he has that maximizes his title winning odds.
I've got OKC as the heavy favorite on a 1+1 with the Celtics and Warriors the others in the mix. As you say, the 1+1 is not a death knell to us as we are in prime position next year while also having another season to see how Durant's foot holds up which nobody is talking about but to me is a pretty decent sized concern. Multiple surgeries plus a minor reinjury of a foot for a 7-foot or close to 7-foot player is something to take seriously.