I think that each film might actually do the thing the other film is regarded for better than the other film? But only for brief flashes or single scenes.
So, The Thing, highly praised as one of the greatest body horror movies of all time, used body horror all throughout to just straight up shock audiences and terrify the bejesus out of people. But I think that the chestburster scene in Alien is actually much scarier and more effective than most of the body horror effects in The Thing, due to its suddenness and how well the actors portray it.
Conversely, Alien is (aside from the chestburster scene and arguably the whole thing with the face hugger) pure psychological horror. It utilizes the tightness of the set design to make the audience feel claustrophobic and filled with suspense even when the alien isnt on-screen. Theres a tenseness to the entire thing that takes hold and never really lets you put your guard down. And yet, the blood test scene in The Thing is imo much more tense and stressful than anything in Alien.
Id say overall I probably prefer The Thing? But I tend to see them as almost apples and oranges within the horror genre. With brief exceptions where I noted, they are mostly trying to do very different things, and they both do them quite well.
Good post.
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