I disagree. I think The Fly's better. Certainly more disturbing, because The Fly is almost like a Greek Tragedy. What The Thing has going for it is its legendary ending, but save for the ending I think The Fly is a bit better. It's a shame that Jeff Goldblum will always be remembered for Jurassic Park and ID4. He should be remembered for The Fly.
It's one of the all time great ambiguous endings. You never know whether or not Childs is The Thing. A lot of ambiguous endings I dislike, but The Thing did it well.
One thing (no pun intended) that both these movies have in common is that they came out in the 80s and were vastly better than their predecessors from the 50s. The Fly (86 version) was better than the original of the 50s. And The Thing (82 version) was waaaayyyyy better than the The Thing from Another World of the 50s.
They're both fantastic films, but I think The Thing is better. In fact, I rate The Thing as the GOAT horror film and one of the very best films of that decade of any genre.