hmm i'm not sure, I do think Shannon's drumming appears more skillful - he has a tendency to play the snare at points that would seem out of step, or unexpected, and I do prefer his drumming style and tehnique.
but then again they are both playing different styles of music, Rob's drums (particularly in early LP stuff) was designed tp not be too flashy - just a steady hip-hop beat. got to hand it to him that later stuff like Little Things is quite a difficult rhthym to pin down and the faint intro is pretty cool. Rob's definitely a good drummer from the Bleed It Out Solos (it's not easy to make those up...) but like brad's guitar he's not trying to be flashy and it's not easy to tell if he's ann epic drummer or not
like I say, different styles of music.