The difference for me has been over longer rides, I feel pretty beat up after 25-30 mi on the 29'r mtb. The geometry and gearing of the gravel bike make longer (30-50 mi) rides SO much more enjoyable. The main surface I ride though is improved rail/canal trail and rural gravel roads with shorter connecting sections of both road and single/double track. Although i don't run a suspension fork or seat post the fatter tires, composite fork and seat / chain stay design of the gravel bike give it some added compliance. I don't have a road bike and use the gravel for the little road riding i do.