I don't know how much this relates to popularity, but fantasies about becoming Batman are less far-fetched than fantasies about becoming Superman.
So what? Fantasies are just that, fantasies. They'll never be real, so really, what difference does it make which hero you fantasize about becoming?
Personally, I'd love to be a mix of both. I have little money, so having Batman's bank account would be wonderful. Plus, I have cerebral palsy and spend half my life in a wheelchair, so having Superman's powers would, again, be wonderful.
But that's the power of imagination. You can become anything you want. Worrying about how far-fetched a fantasy is should be the LAST thing on anybody's mind.