From Garrett Reisman:

Hi Jude! The Shuttle was a bit bumpy during the first stage. During my first launch on Endeavour, I was seated on the middeck so I didn't have much to do during ascent. I did have a kneeboard though and I took a writing sample during the first stage and my handwriting was even worse than usual! It was much more legible during the 2nd stage. But I would say that the vibration wasn't really that bad even during the first stage. It was comparable to flying an airplane in light to moderate turbulence or being on a typical motion-simulator ride. Nothing too crazy.

As for SLS, I have no personal experience on that thing. :) Nor have I seen any analysis of the random vibe environment. But I don't think that the extra segment would make it worse, it would just make the SRB portion of the ascent last longer. Plus the additional mass of the SLS compared to the Shuttle would likely give you more inertia to reduce the amplitude of the vibration, so I think the ride would probably be quite nice. But again, I'm just speculating here!