Show them in the same theater, see if they fight.

A comparison of the two films would be an interesting exercise. I don't know how many elective courses I passed writing things like that :-)

Without doing anything resembling a formal analysis, on first blush I'd say there are some significant differences. In WotW we actually see most of the things that are only hinted at or described in Signs...most notably, endlessly hoofing it away from a nasty peice of alien technology that is hell-bent on turning you into a small pile of fireplace scrapings. The difference in focus makes the films hugely different in pacing, look and feel. Signs is often leisurely (though imho never slow), but WotW is downright frantic.

In Signs, the aliens themselves are secondary to the main plotline--namely, the main character's rediscovery of faith. In WotW, the aliens (or rather, their machines) *are* the main plotline, though the relationship between the Cruise character and his family comes in a close second.

M Night Shyamalan is one of my favorite filmmakers. I think the guy's a genius in the same way Hitchcock was a genius. When asked once about the potential impact of a plot weakness in one of his films, Hitchcock replied that if the audience is thinking about those things, he'd failed. From a science-fiction standpoint, Signs has at least one obvious weakness, but for me it just doesn't matter--the story overpowers it.

WotW is awesome fun, but even though the acting is really pretty good, the reason I'll eventually buy the DVD is so I can watch the alien machines. The reason I've already watched Sign probably half-a-dozen times is for the acting and the story.