The Asus looks very sleek and nice, and it's relatively cheap. What worries me though is the stand. It looks kind of fragile (I wouldn't want to send it flying in a fit of rage or something...) and I'm not fond of it being at a permanent tilt. I like to keep my monitor at a 90 degree angle to my desk.
The alienware is considerably more expensive and has roughly the same specs (though it is slightly smaller--this doesn't bother me because 21.5 inch is enough for me anyways). The selling points are that it's more traditional (but with a great design) so it will be able to be at a 90 degree angle, and best of all it has height-adjustment which I happen to like a lot.