Both will work. It all depends on what you are drybrushing. I usually use a wider brush when drybrushing, when l dont want the effect in too many of the shallow crevices. Sometimes when the surrounding areas are already painted, or you want to concentrate on a particular detail the smaller brush works best. Even smaller brushes have advantages some times.
If the bristles of the brushes are stonger its easier to get defined lines, but a wide brush with soft bristles is better if you want to add texture or fade areas by rubbing in a colour.
Dont be afraid to experiment with different size brushes, with different bristle characteristics. I have 2 brushes ... 1 and 2 inches, that are great for diorama and items like wheels where the width helps get the effect that is needed.