As you people already watch Watchmen and The Dark Knight and these two films don't just rely on that old standby -- being "dark" and "edgy" -- they also take their subject matter into truly adult territory, not just in terms of sex and violence (as in Watchmen in particular) but also in theme. The superhero movie has grown up this past year, and it's Snyder and Christopher Nolan who we can thank for that.
So tell me which one is better and why ?
Well they are very different considering Watchmen is an adaptation of a gn and tdk is an adaptation of a superhero series. tdk wasn't perfect but man the plot, actors, and action was something special. Where as Watchmen has many flaws, it also didn't get close to the brilliance of tdk (but the flashback scene with Dr. Manhattan was incredible.) Watchmen is pretty good, while tdk is excellent.
I can understand why people would, and will, say Watchmen over The Dark Knight (for much the same reasons people say Batman Begins over The Dark Knight) 'cause Watchmen was much easier to watch.
The Dark Knight is a film which you have to be emotionally invested to watch. I know from all the people I've spoken to about it you have to really pay attention and watch it. Many people were sort of drained after seeing it for the first time. You have to give in to the tension, the horror and the intense emotions of the characters. Watchmen doesn't have that.
Watchmen is much slower, much more picturesque in its presentation and doesn't require the level of emotional investment that The Dark Knight does. Watchmen, as a film, comes across as being more fun, more popcorn, something which you cannot call The Dark Knight. Yes there are intellectual themes to mull over, but Watchmen has all the blood and sex which you can enjoy on a more shallow level allowing to ignore the deeper layers if you wish. With The Dark Knight you don't get that shallower, blood and sex, level. You are confronted with the problems of the characters, the themes of the film, and the moral ambiguity of the characters.
It's not that Watchmen is a worse film for not being as intense as The Dark Knight, nor The Dark Knight for not being as popcorn, it's that Watchmen is softer and easier to watch. It's closer to traditional adaptations of comics than The Dark Knight. That may mean you'll watch it more but I think The Dark Knight will have a greater impact on film, and on comic book adaptions, than Watchmen will.