First thing to remember: the short story is not the screenplay.
The short story is not bad, but it's not as good as the movie.
King's story was creepy in its own way, and it deserved to be made into a movie. However, the screenplay makes Mike into a real character, one the viewer wants to get to know better: a writer whose own tortured soul was mixed with the unrelenting evil of the hotel room. The movie wasn't perfect: it could have been a little shorter, and the hotel room scene went beyond the point of exhausting to making me wish it was just over, for the love of Pete (and not in a good way). Having said that, I wouldn't really opt to change much.
I also preferred the way the movie ended to the story's ending. (Important tip: don't watch the alternate endings on the DVD. There's a reason they weren't chosen as the final ending.)
I am thrilled that the producers chose the actors they did for the movie. King's descriptions do not jive with the actors, but I preferred the actors to those described in the story.
If I had to choose only one, watch the movie and skip the story. You lose nothing by seeing only the movie, which captured the good parts of the story. And if you are going to do both, watch the movie first.
However, skim through the other stories in Everything's Eventual and let me know if you think King has started getting reigned in (by himself or an editor) or if he still is the 1,000 pound gorilla who isn't edited enough. I read a couple of the other stories, and they're not bad — but life is too short (and there are too many good books) to read "not bad."