The Ksyrium SL is absolutely a stiffer-riding wheel, particularly up front. That paper-thin aluminum extrusion Shimano uses beneath the carbon seems to be the key as it's noticeably smoother-riding than the vast majority of wheels out there.
Torsional stiffness seems quite comparable between the two though the Shimano one *maybe* gets the edge - tough to tell absolutely without a test jig.
In addition to the comfort thing, though, cornering traction on the Dura-Ace wheels is awesome and very confidence-inspiring. Especially on bumpy tarmac, the softer rim is better able to keep the contact patch glued to the ground.
As for bearing smoothness, I'd say if anything the Ksyrium has the edge if side loads aren't involved. Though they definitely require far more regular servicing than Shimano, virtually any bearing issues I've seen with Mavic hubs over the years (both in my current capacity at CN/BR and nearly 15yrs as a shop mechanic) have involved users overtightening the preload collars or letting the freehub bodies run dry for prolonged periods of time.