Iron sights look badass if done right, and are a lot more realistic, but they limit sight and can’t have things like the episode’s flashlight and health meter on them. Crosshairs don’t look as cool and aren’t as realistic, but iron sights don’t seem to fit in games like HL2. Iron sights require slow movement for accurate shooting while crosshairs don’t.
I personally like both. I think iron sights work best for realism games and war games like COD and Far Cry while crosshairs work best for the sci-fi genre.
The one game I think could have used iron sights that didn’t is Fallout 3, but I guess they didn’t want to have to make that many more animations for every gun.