To make it short:
Now I use (again) SyncBackFree
Before (up to 2017-02-01) I used FreeFileSync
o) good overview on the changes before the sync (best user interface among the compared applications)
for example switch on/off equal files
o) versioning (maybe the greatest advantage)
o) portable (now only available in donation version)
o) tries to install additional software (not in donation version),
sometimes Avira Antivirus blocked a downloaded version (move to quarantain) (as shown in the picture below)
o) donation version *costs* € 5,00 per month (it is not a one-time donation)
I think € 10,00 per year would be enough.
Recently (2017-07) I discovered another alternative: Alwaysync
The functionality seems to be very good. It offers a versioning policy. That is an advantage.
The overview of the processed has another philosophy: it shows the files grouped according the suggested processing: changed files, questionable files …
I’d prefer to see all files (according the folder structure) and show/hide categories.
FreeFileSync worked excellently, but the above mentioned disadvantages let me switch back to SyncBackFree.