For me personally I prefer Watchmen, The Spirit is fun I enjoyed it alot more than I thought I would.But nothing can beat the amount of detail and the sheer depth of storytelling Alan Moore gave in Watchmen, plus its the only comic/graphic novel /collected series or whatever you fancy that made it too Time magazine's 100 Greatest English Language Novels from 1923 to the Present. Plus Stan (the man) Lee said it was his "all-time favorite comic book outside of Marvel."
There are single issues of The Spirit which I prefer to Watchmen, but it has no continuing story arch, so it doesn't really amount to much as a whole compared to Watchmen.
Watchmen is better overall as a complete work, but of what I've read of the Spirit (the "Best-of" collection) kicks major ass. The artwork is a little better in Watchmen than Spirit, which is understandable since it's different generations of artists, but the writing is probably on par. Will Eisner is a genius.
Once again the player is jerking me around, so im Halfway from snapping, this week I got Member Jordan Cobb as a guest and we went on for great lengths (15 more minutes than usual)
enjoy