The Thing is one of my favourite movies.
As far as the end goes, I think as well that neither of them is the Thing. In the commentary, Carpenter says more than once that the movie is about trust. At the end, both expect the other is the Thing, but neither is IMO, and that makes it more poignant .
Almost every question has been answered about who was the Thing when, and other mysteries in the movie over at the fan site outpost31.com's forum.