Bruce likes X's apartment. Not that there's anything wrong with Wayne Manor per say, but it's hardly what one could call...intimate. It's home, and it's his parent's home, and it's the Wayne house, for all those who don't live there. It means things to different people, whether good or bad.
For him, it's just a bit huge. Great when he needs space to work out, or build the Batcave, or throw elaborate parties. Not so great when you feel like watching a movie with your girlfriend. Somehow, the movie theatre there lacks the personal touch. It's far nicer to sprawl on the sofa in her apartment, and share popcorn (and occasionally get attacked by cats).
Maybe Wayne Manor could use a cat. He's thinking it wouldn't make that much difference, though.