If you are going to CCW it, the G19 is smaller.
I prefer the G19 to the larger G17 frame. It is easier for me to carry.
.40/.357 conversion is just a barrel change, all the other parts are the same.
9mm, technically requires the extractor/ejector/barrel to be changed. They are different between 9mm and .40/.357 and while it might work to just change the barrel, I wouldn't want to rely on it for anything other than goofing off at the range and I can't advise you to do that as an Armorer.
I prefer just 9mm myself. The other rounds are harder on the guns and much more expensive to shoot for little advantage.
I have owned the 34 in the past. It was a nice shooter and pretty much set up for competition from the factory. (3.5 lb connector, extended slide stop, extended magazine release).
I don't like the extended slide stop or release personally.
I am no longer participating in gun forums.