You don't HAVE to play either one. Yes, the mechanics are the same and some of the game play, but there are different aspects to B*. For example, strategy is required for certain bosses and to access rooms. Ie...u have to be a certain class to open this door or a higher level of another class to get through, random chest spawnings, things like that.

It's bound to be similar due to it being the same company, but they are trying to implement things that are specific to each game.