Over-sealed surfaces can benefit from a spray application - the applicator can leave a thicker coat than with a squeegee. When using a squeegee on a smooth surface with very few voids to accept the sealer, the squeegee leaves very little sealer. The right spray technique can leave a better coat of sealer.
Porous surfaces will always cover well when sealer is hand applied, but can suffer if too light a coat is applied by spray. With spraying there are more variables that come into play, spray tip size and spray wand speed determine how thick the sealer goes on.
Professional sealcoaters can achieve great results with both processes. Hire a reputable contractor and trust them to use the process that works best for your driveway.