Preacher is bullshyt and about whether there is a god filled with pathetic writing about American culture with people who aren't even from the U.S. Watchmen has so many themes and works in so many levels in so many different ways.
Preacher: "hahah arseface" "Lets randomly have sex and get drunk"
Watchmen: "Existence is random. Has no pattern save what we imagine after staring at it for too long. No meaning save what we choose to impose. This rudderless world is not shaped by vague metaphysical forces. It is not God who kills the children. Not fate that butchers them or destiny that feeds them to the dogs. It’s us. Only us. Streets stank of fire. The void breathed hard on my heart, turning its illusions to ice, shattering them. Was reborn then, free to scrawl own design on this morally blank world."
Never compare the two.
Preacher is good don't get me wrong but not anywhere near the best.