Batman’s more interesting because he’s a super-capable human who is more vulnerable and relatable and in theory his stories involve more engagement with figuring out what to do about situations, which I’m interested in.
Superman is less interesting to me because I don’t believe in his powers, they don’t make sense to me, the universe gets bent by sloppy imagination in ways that don’t make much sense and so aren’t very interesting to me. Also he’s too super. He’s almost impossible to hurt or resist except by avoiding his attention. I’m not very interested in most of that. The most interested I was in a Superman story was the second Christopher Reeve movie (Superman II, I think) where he had to fight three others with his same powers but less experience using them and fewer moral restrictions. I liked some of that one. Still not really believable or particularly interesting, though.
My favorite super-heroes are the ones who don’t violate my skepticism too much, and who have some sense of humor and are likable. I like Kick-Ass and Hit Girl, because they feature semi-realistic logic-based violence and they’re all mortal and what happens makes at least an action-movie’s amount of sense, and it has a lot of humor and consequences of violence and choices in it.
I liked the first Robocop film similarly because it was semi-plausible and the violence was pretty gritty and horrible and it made some sense and had a sense of humor and made pointed cultural commentary. The later ones were all crap as far as I was able to make myself watch them, though.
That is, I like superheroes who are heroes in relatively realistic situations. The more disconnected from reality, the less I’m interested.