Beveled because it adds a depth and more interest.

Flat will stay in style longer and appeal to more people if you ever sell.

As you seem to be going for a clean sharp modern look i'd definitely go all the way with the flat sub, it'll look classier with the shakers and will allow the countertop to express itself better. Besides, the bevelled has always looked vaguely gross to me but that is of course a matter of personal taste.

I have learned over many years' and lots of houses….less is more….definitely use the flat!

I always use flat I think they are timeless.

Flat

flat. the beveled looks like the concrete blocks used to build garages in the 1930s in my area. (boston).

Really like my beveled in white thassos...it covers an entire wall (to the ceiling) and adds texture and character to what could be a very flat space otherwise.

I would go with the flat with a light grey grout. Timeless, classic, lasts forever. I think with under cabinet lighting, using the beveled you will have reflections you did not anticipate. Now you need to decide whether to use the matte finish or the shiny finish.