Go glock, but go gen3.
I'm not a big fan of the gen 4, but that's just me, you may like it.
To offer a counterpoint, I do prefer the Gen 4, but all my love is for that adjustable backstrap.
In the Glock vs M&P debate, there's really no wrong answer.
If you get a 357Sig then you would get a 40S&W barrel to work with the slide.
from what i read, you can switch both 40 or the 9mm barrel...
Glock is the original and the one that all others copied...in one way or another.