I have been testing both with Backblaze B2 and am still trying to sort out which I will adopt. One big difference is speed/deduplication. They take two different approaches. Duplicacy by default relies on file size/time stamp differencing unless you specify the “-hash” tag which is not the default while Duplicati chops files into much smaller pieces for more efficient data reduction. These differences make the two products hard to compare. Here is the results from a test uploading 3GB of actual data with both solutions.
Duplicacy: Upload time ~45 minutes
Duplicati: Upload time 1 hour, 9 minutes.
Duplicacy: ~30 seconds
Duplicati: ~2:40 seconds
In short Duplicacy seems much faster although Duplicati is storing about 200MB less data. To be 100% clear, I am running this on a small single board computer running Ubuntu dedicated to this purpose, and so compute power, memory and bandwidth are limited. (Your performance is likely much faster.)
I really like the Duplicati GUI and there is no GUI for Duplicacy on Linux; however, I find the performance difference significant.
I will continue my tests and welcome any feedback.