I'm wondering the same as I design my gantry hoppo, a top & bottom mounting for the rails I think would provide a slimmer gantry so the router bit will be closer to the gantry but it also seems to sacrifice some gantry stiffness when compared to side mounting the rails.

Assuming all other materials and dimensions etc stay the same, does a top/bottom mounting of linear rails give better real world performance or side-mounted ones?