Ah, what a question...
its really more of a question of how wide a tire do you want?
I have some 32mm tires that are some of the fastest made and are a ball on my commute.
I have some 60mm tires that are also some of the fastest made, and they are super cush, roll fast, feel slower (i.e. more comfortable). I
I like the cush in rides that are <15mph, the responsiveness of the 32mm tire in rides over 15mph (yeah, there is some overlap).
In race type condtions, the 32mm tire spins up faster and probably is more aero and has a higher top speed. Usually not a big deal when commuting though.
The point of 650b (i.e. 27.5) is to get a wider tire with the same diameter as a 700c (i.e. 29" tire). Certainly there is a more limited choice of slick 650b tires compared to 700c, but there are some good options out there.
(tires: 60mm x 650b Schwalbe slick, 32mm x 700c Conti slick).