ok, if you want to talk a college professor into respecting comics, or at least not looking down on them, you don't give him Preacher or Watchmen.
Watchmen is still superheroes, and he wouldn't walk into it with an open mind because of that. You'd be better off with V for Vendetta if you want to go with Moore. V is BARELY a "superhero", with far deeper tones that come out more directly.
Preacher takes too long to get to the undertones, and all he'd see when he started reading would be sex, violence and vulgarity.
The best things to approach him with would be Road To Perdition, Y-The Last Man, certain key issues of Sandman (Three Septembers and a January, Ramadan, Men of Good Fortune, etc.) and the like.
He's an English Professor and it's a good bet he roles his eyes at all fictions which contain elements of fantasy, so if you want to open him to the medium of comics it should be through comics with a minimum of fantastical elements