SyncThing needs to be installed on both; each instance scans its own content and compares notes with the other(s) to detect changes. This is different to FreeFileSync where it goes on one machine and scans both its own content and the remote content to detect differences. The former is better in slower networks, busier networks or involving large numbers of files (the issues compound each other), as it involves much less back-and-forth of network traffic, but FreeFileSync can compare a surprisingly large number of files on a fast network (e.g. about fifty thousand files per minute when my 1Gbps LAN is idle) and I feel its GUI is rather more user-friendly.
Whichever option you go for, I'd suggest creating a test pool to trial it before committing your real pool - and you could make two test pools and try both.
