To be honest, I am NOT the world's best (fretted) bass player (let alone fretless

) so no fretless clips (for the time being at least).
There was no big learning curve really over the fretless Dingwall. I have been playing fretless since 1999 at the same time that I started playing my "Uber Alles" Prima as well. So for the past few months I've been gelling the two worlds together. It takes a bit of attention in the beginning but after a very short while it all comes together
As far as choosing between the Z3 and SJ5 in a fretless form, I cannot firmly comment. The SF I's that came with my Z3 had great singing mids, but the clarity of the B and E strings did not have the clarity that I dearly love. From what I know the SF II have rectified this issue. So I am waiting for my SF II's to form an informed opinion. In terms of plain physics, the "shorter" SJ5 will have a "sweeter" and more mid-specific tone compared with the "extended" Z3. Once the SF are the standard pickups now, it's a matter of choosing the ergonomics that suit you better over the tonal approach you are after