Both on the front tends to reduce spacing between them, hence more leverage for a given cutting force - but this leverage is in the plane of the screw fixing. It also pushes the spindle out from the gantry which could increase the twisting moment on the gantry. However it does increase clearance under the gantry and can allow a greater range of movement in Z. Top and bottom has greater spacing and hence less leverage on the rails, but it's then sideways which might not be so good unless you've milled slots as Joe has in this build. And you'll lose something like 30-45mm clearance under the gantry depending on the size of rails & thickness of mounting plate you're using. So there's pros and cons both ways, in the end it will all depend on your design as a whole.