I'd go with the G19. Since you're already invested in a 9mm, you will be able to use that ammo in both weapons. And 9mm is cheaper to shoot than .40S&W, making more practice shooting possible, since dollars spent on 9mm rounds will net you more ammo than the same amount spent on .40. We could debate here all day the merits of each of those calibers, but in the end, I consider them to be equivalent.
If you want a bigger caliber, my money is on a .45ACP. Forget about expansion, fast/light vs. slow/heavy, temporal cavitation, etc. All modern hollowpoints will expand reliably in modern firearms. At the end of the day, in a defensive handgun, you either want lots of rounds to put downrange into your intended target (9mm), or a bullet that makes larger holes in the target, thus increasing blood loss and incapacitation. The .40S&W was designed as a compromise caliber, to fill both of these roles simultaneously. And while it does both to an extent, it does neither as well as the original.
Beyond a .45, I want a 12-ga shotgun or a centerfire rife of .30 caliber or better.
No matter what detractors may say, a well-placed shot from a 9mm will stop an assailant just as effectively as a well-placed shot from a .40 or .45. "Spray and pray" is much less effective.
