Alien VS John Carpenter's The Thing
On the one hand, Alien has a better looking monster and Lance Henrikson. But The Thing has got more crazy ass special effects for the monster and Keith Fucking David.
Cons for Alien: I've never seen it. And it's had multiple shitty sequels.
Cons for The Thing: It's a remake of a shitty 1950's movie (but has had no shitty sequels thus far).
Winnar: John Carpenter's The Thing
Mostly because I'm watching the shit out of it RIGHT NOW!