Ermm to me by arranging the profiles at the top of the 160 side pieces your wasting 80mm of height that could be used to lower the gantry height and increase stiffness or keep as it is and gain clearance.
The gain in having the motors under the bench isn't worth wasting stiffness, also moving the motors to the outside will shorten the belt length.

Also, the slightly elevated sides put the rails up higher and stop debris from hitting them directly and building up against the rail.