If it's going to be a range gun, then the 9mm will be cheaper to shoot, and, probably more accurate than the .40 S&W. With modern loadings the 9mm will be fine for a home defense gun as well. The .40 operates at the high end of the pressure scale, is generally less accurate, recoil is snappier, is more expensive to shoot, and does little to no better in the stopping power catagory. IMHO, a shotgun is the best "home defense" gun anyway.