The DA42 has slightly less headroom than the PA28. The DA42 sim has marginally less than the aircraft (the canopy actually rested on top of my head, but wasn't a problem as the sim had no motion to bounce it around and bash my head off the canopy!). The DA42 seats don't move at all, only the pedals, so height can't be adjusted, but it's a very smooth ride. The PA28 is more prone to bump around in turbulence, although I only ever bumped my head while sitting in the back. In turbulence, it (PA28) was fine in the back while sitting at an angle. Never actually back-seated the DA42 as with full tanks on take off, it was difficult to plan to be within weight limits on landing, with me in the back and a full sized instructor and cadet in the front. That wouldn't have been an issue if you were up for a couple of hours, though, burning more fuel.