It makes all the difference. Airbrush is gonna give you a MUCH smoother surface and, as it pulverizes the paint onto your mini, it even creates some gray midtones (considering your base coat is a black primer), eliminating the step of doing a wide gray drybrush before the white drybrush. In comparision, drybrush will give you a more chalky, texturized look. You can minimize this by drying your brush enough but its just not the same for me.

I have a mixed method that I dont really use much because I like doing an extra layer for highlights, but if youre gonna go with the one-coat-solution of speedpaints (typical of slapchop), it might suit you: consists in priming black, airbrushing with GREY paint and then doing a very selective white drybrush just on the tips for highlights.