Too bad nobody had any input on my question. :-(
I still do not know if there are any advantages or disadvantages of using the XLR output unbalanced in comparison to the unbalanced headphone out, but I guess neither.
The dip switch on the Babyface Pro FS needs to be set to +19dBu normally. The +4dBu setting results in a relative low output to connect to typical hifi-amps.
If someone wants to know which cable to use:
I do not know for sure but have the impression that the cables you can purchase will have pin 3 connected usually. I bought the Cordial CFU 3 FC (XLR to RCA) cable and needed to disconnect pin 3. To do so one needs to open the XLR connector of course. For this specific cable that was really hard, because the plastic shell/screw on the back of the XLR connector was so tight that I needed to use a vice and a (pipe) wrench – using wood and textile to protect the metal connector and plastic screw. After the plastic screw is broken loose one finally can easily open the XLR connector. Cutting through the cable of pin 3 is easy, re-assembling is not super hard, but a little bit fiddly.