I'm not a fan of .40, but if you like your sd40, the shield .40 is really good shooting gun. There's a million options out there, my wife carries a .357 snub as do I, I ordered a cz compact clone and that will be my carry gun next week, as long as it proves to be a reliable pistol. You just need to go out and shoot some guns after you figure out what your budget is going to be. You have size, weight, power and capacity to think about, but none of those things mean squat over reliability. So when you find "the" gun, research and research to make sure its not a p.o.s.' I made that mistake on my first carry gun, the sccy cpx-1(I don't want to knock them down, I hear they are very reliable in their new generation of guns)
Good luck, let us know your budget, and you will get a zillion great suggestions from the TFL guys