Couple of other things to consider:
1) 40mm is a different view. To me, it's more natural than 35mm and 50mm.
2) The 40mm STM images look more "rounded", to me. The 35mm USM IS images look kinda "flat" and 2-dimensional in comparison, to me.
The old EF 35mm f/2 version w/out IS delivers much more natural looking, "rounder" images than either of the above, for my taste.
I've owned the 35mm f/2 IS USM for several years. This is the first time I've heard of anyone favoring the older 35mm f/2 over it.