Next One?

G32 and 40 barrel or G23 and 357 barrel, makes no difference, which ever you prefer, and no you don't need anything else, just the barrel switch and you are good to go

 

Why not just feed your G19 some +P+ ammo, same difference as 357Sig IMHO. Also, 9x19mm+P is just as effective as 40S&W or 45acp so unless you've just got cash burning a hole in your pocket that you just have to spend on a new Glock I'd look at getting more mags(while you still can) maybe another holster or two and some good quality Self Defense ammo and call it a day.
Other wise sell your G19 and get a G26 for CCW and a full size G17 for around the house but still, you're G19 can pretty much Do It All IMHO.

 

The 357 sig does get more muzzle energy and velocity than a 9mm +p+, and the 357 sig holds a better bullet drop out to 100 yards, although I don't see many people doing alot of 100 yard shots with a compact pistol

 

diversity is a good thing

The G23 is one of my favorite handguns of all time, and the 357 sig is a fun gun to shoot besides being an effective man stopper. If you are thinking mid size pistol in both 40 and 357sig, you can't go wrong with either the G32 or G23

 

It's slight, but its there, certainly controlable with the 40 and 357 sig in the compact anyway. If you are accustomed to standard load 9mm it may take a bit of shooting to get used to it, but its not bad by any means, just more muzzle flip