There is an interesting discussion comparing Duplicacy with Duplicati: Duplicati 2 vs. Duplicacy 2 - Comparison - Duplicati
Performance wise, Duplicacy is about 2-3 times faster than Duplicati with the default settings (3:09 vs 9:07 for a small data set, and 0:27:27 vs 1:12:40 for a larger one). By disabling compression and encryption, and applying an optimization on the hash function, they were able to achieve the same or even slightly better performance (than Duplicacy with compression and encryption), but the CPU utilization was still significantly higher.
Those tests were done using one thread. Duplicacy already offers the -threads option for multithreaded uploading and downloading, while Duplicati currently doesn’t, and it is unclear when it will be available.
Feature wise, Duplicati comes with a web-base UI and is more feature-complete than Duplicacy. It also has a larger number of storage backends. However, Duplicati lacks the killer feature of Duplicacy, cross-client deduplication, and it doesn’t look like it will ever support it.
In my opinion, Duplicati’s main problem is the use of the database and the aggressive approach to attempt to ‘repair’ it (as pointed out by several users in that thread). More than one new Duplicacy users mentioned that the database corruption is the main reason they gave up on Duplicati. Their developer argued that by taking an aggressive repair approach one will be able to identify storage issues sooner. In Duplicacy we took the opposite way – we assumed that all the cloud storage can be trusted and we based our design on that. In cases where a cloud storage violates this basic assumption (for example, Wasabi, OneDrive, and Hubic), we will work with their developers to fix the issue (unfortunately not everyone is responsive) and in the meantime roll out our own fix/workaround whenever possible.
Having said the above, I applaud their collaborative efforts to start and expand such as discussion. Both sides can learn from each other and have something different to catch up on. A comprehensive and insightful comparison like this will only advance our understanding and make both software better in the long run.