I am just a Student not any marketing expert. I am making games for more than three years. So to balance between A and B is to just give the audience a good glimpse of one or two key features that the game would extend later on, and for this a demo gameplay of 5-10 minutes is a perfect timing or if it had various levels then show them the intro of the story and the first level. Show the players that the features have possibility to be extended and do not get bored over time. Show them interesting things also like artwork, illustrations or music that also enhances the quality of the demo. I hope you've got your answer.
By the way I am a solo developer doing all the things alone.