http://www.need-4-speed.com/pst.htm Go carbon!
For a given material, and given outer dimensions, the maximum strength is achieved if the rod is solid (not hollow), but this also takes a lot of weight.
For a given material, and given weight, the maximum strength is achieved by a hollow cylinder, that is of larger diameter than the solid rod of that weight.
Most of the strength of a cylinder comes from the outer portions. I think the contribution goes like the cube of the radial position. So, if you took a solid rod and drilled out a half the volume from the center, you do not lose half the strength. You only lose about 10% of the strength, but you've saved on half the weight.
So, strength to weight ratio is better for a hollow pipe than a solid rod.
I knew there was something to this just could not remember. So if you have a 2 Drive shafts same LENGTH WIEGHT AND MATERIAL the hollow will be stronger ( but with > Dia)
SImilarly 2 Drive Shfts same LENGHT DIAMETER AND MATERIAL the solid will be stronger but also alot heavier and like SILVERBACK pointed out will have different rotational properties with slight increase in stregnt but aSL more weight.
So essentially everyone is right here.