I prefer resilio because it seems more straightforward to set up. Add a shared folder to it, then it can provide a code to sync that folder to other machines. Then all the peers can sync files from any of the other peers. (I actually use to paid version, so that adding a new folder becomes available to sync everywhere).
I’ve recently tried SyncThing and first I accidentally created separate sync folders with the same name on different devices. I also didn’t realize that I had to set up which peers to sync with on each device, so some files did not sync to one phone because it only synced with a laptop that was offline. Also, the mobile app is limited until you use the “open web view” option that views a web page allowing more features to be edited (e.g. I could not change a folder to be bi-directional syncing on a phone and kept having it overwrite files with old ones until I found that setting).
However, last time I tried the resilio phone app (admittedly a few years ago), it worked by pollng for changes which drained the battery. Synching has not done that, it probably uses change notifications. So, despite the more confusing setup, I’ve started using syncthing for syncing to phones. Once I’ve become more familiar with it, maybe I’ll be happier to use it more.
But so far, in my limited experience syncthing is a little buggy - I’ve just checked my phone now and it’s just stuck on “syncing 91%” for the one small folder that I’m using. And the mobile app having two different interfaces is a strange choice.
This sort of problem, btw, is why I mentioned installing winmerge! It’s a great tool for sorting out merge conflicts in files.