I liked both of these movies. Fight Club is still seen as this cool, kind of underground movie that has a few surprising twists that'll catch you by surprise if you're not really paying attention. It's probably my favorite movie featuring Brad Pitt. I'm not a big fan of Pitt, the guy can act but I think a lot of his success can be attributed more to his handsome features more so than his talent.
When it comes to 12 Angry Men, it's just all around great acting in a minimalist setting. It's all about these 12 ordinary men put into what can be an extraordinary situation: deciding whether or not to send someone to the electric chair. As a corrections officer, this movie spoke to me the first time I saw it, which was only a few years back. I caught it on TCM one night. I've seen lots of people convicted by juries for numbers of reasons: some did it as a means of venting frustrations on something in their own lives towards the defendant, some are there that don't want to be there & just want it to be done as quickly as possible, some are able to vent their own prejudices, etc. All of these reasons are explored to some degree in the movie. The movie never explicitly shows whether or not the young man is truly 100% innocent or guilty as it's really impossible to know, something that's actually brought up by Henry Fonda at one point. But the movie focuses on looking deeper than just beyond what you see & hear in a courtroom. The actual truth, or what you feel is as close to the truth as you'll ever be able to know, lies deeper than just what's on the surface.
Both movies are good but, for me, I went with 12 Angry Men. It's almost as low budget as it gets. No big fight scenes, no special effects, no shoot outs, no nudity, no sex, no over the top violence. Just a simple, real life setting that tells a great story that features great acting. At it's heart, it's what storytelling in movies is all about at its purest.