I would say they are a bit more supported on the front edge because they have the vertical face of the section they sit on directly underneath. If they are in the middle of the section they are in the middle of the unsupported membrane of the section. It also minimises the offset from the spindle to the rail (a little bit).

If they are steel they would of course need to be set back a bit as there will be a radius along the edge.

In terms of performance comparisons this confirms what I said earlier in that the vertical stiffness is easier to achieve than torsional stiffness. To get the same deflection in both directions you need a lot more material resisting the torsion/moment than the more straightforward vertical load, hence the horizontal L.

But I would also say that the vertical and horizontal L shapes would both give good performance in practice, and that in the end it will come down to whether the horizontal L leads to unacceptable loss of travel in the X axis for any given design.