I own an Axe II XL+ and will probably be buying a Kemper soon to add to my arsenal.
All I can say is, when top artists and engineers are using both units in their studio and on the road, it says something about the accuracy and authenticity thereof.
That said, I haven't used a Kemper heretofore, but my impression is that the amp models may sound slightly more accurate / authentic right out of the gate, though that isn't to say the XL+ can't be made to sound practically indistinguishable with a little (or a lot) of tweaking. I don't know and won't know until I get my hands on a Kemper. My plan at this point is to run the Kemper into the Axe in order to use the XL's effects with the Kemper's amps, though that isn't to say I'll be abandoning the Axe's amp and cab sims. I won't. It's mainly because Axe-Edit is awesome, and will simplify adding effects regardless which amps I use.