I don't understand why you are insisting on using the least amount of brushes possible. All those brush faces are split up into triangles during the compile anyway, and grouping makes it easy enough to treat multiple brushes as one, anyway. You aren't saving wpoly; the only possible result might
be a slightly reduced compile time because of slightly less faces for the compiler to null off. Maybe.
Just work with triangles from the beginning, it's so much simpler.
I don't know exactly what bevel does, its been too long since i've used it. But i do remember people saying it was good.
Bevel is like null, but it also causes the face to ignore clip nodes during compile. In other words, the player can walk through a beveled face. It's useful for maps with lots of complex geometry, IIRC.
Bevel was also introduced much later than null, I think.
Edit: Shape #2 shouldn't be made out of one brush in the first place. That's literally one of the first things a mapper learns.
And just for good practice, shape #1 shouldn't be one brush, either. If you insist on not using triangles, you will eventually do this by accident: This will not
compile, and you will go insane trying to find the problem.
Edit Edit: To answer your original question, neither is more optimal, but it's safer to just use triangles/wedges/tetrahedrons.