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In torsion, the center of the bar contributes little to the torque, as it has no leverage (radius from axis of rotation). That is why a solid bar is only a little better than a hollow bar of the same diameter, for torsion. For bending, actually I believe a hollow bar is much easier to bend than a solid bar...
Link to some info on the subject:
http://www.hotchkis.net/pdf_files/HollowSwayTech-D.pdf |
That is a very informative link. The solid bar would probably bend first. And from that link it says that the slightly larger dia. hollow bar is stiffer, weighs less, and sees the same amount of stress.
For the average street car the difference between hollow and solid bars designed by reputable shops would be unnoticeable.