Well, you haven't mentioned a price range. CZ and Sig are both solid choices, and would be my first recommendations. Bear in mind, your 9mm is hardly underpowered.
I don't care for the .40, mostly because of the "snappy" recoil that others have mentioned. By snappy, I mean the muzzle jumps far too much for my tastes, and it takes a second to regain sight-picture, something which does not happen with the .45. The latter is more of a push than a snap. That, and I've never really gotten much accuracy out of the .40.
My advice is not to shop by caliber, though. If the gun feels good and shoots well for you, you should be able to master it, regardless of caliber.