When determining if you should go for a mountain bike which has drop bars or a gravel bike, one of the key considerations is going to be aerodynamics. As the course conditions become for example more smooth and faster, aerodynamics plays a more and more important role making gravel bikes more suitable. However, on rougher and slower courses where rolling resistance is more important, disadvantages of an aerodynamic paradigm may be compensated by using a mountain bike with wider tires that have drop bars.