The main difference is that overlaying is a structural repair that will extend the useful life of your asphalt surface. It involves the patching (if necessary) and resurfacing of the entire surface with a new overlay of 1 ½" to 2" of hot mix asphalt. Sealcoating is not a structural repair. The sealcoat protects and extends the life of your asphalt, but does nothing to correct structural flaws. Sealcoating, accordingly, is substantially less expensive than resurfacing your drive.