Cleaner bass. In stereo, each speaker will have its own bass "tune". One subwoofer means your bass is in mono, and has to work harder to play both tracks.
That said, your budget matters also. For example; one $1500 sub is generally better than two $750 subs. Better to get the $1500 sub and then save up and buy the second one later. (Or whatever your $$$ budget is).