TL;DR: Use Syncthing over this unless you have an iPhone and don't want Resilio Sync for some reason.

So I use Syncthing for file sync between devices. Some spelunking in Duple's FAQs shows the following blurb as to why to use Duple (a paid offering) over Syncthing (a FLOSS offering), along with my own point-by-point comments / pseudo rebuttals.

-Syncthing is P2P, so you get the disadvantages along with it e.g. all your devices need to be turned on at the same time. If not, you get a desynchronisation between your devices and create conflict. In Duple's FAQ page it has one reference to offering File Conflict Resolution.

Syncthing does not require all devices to be on at the same time. If a device is offline, when it comes online, it is brought up to speed by the other peers in the cluster. Unless duple has done some serious magic, it will likely require devices to be online to sync as well. According to https://docs.syncthing.net/users/faq.html#what-if-there-is-a... , if you edit a file from two different devices at the same time, the one with the older modified date gets renamed to <filename>.sync-conflict-<date>-<time>-<modifiedBy>.<ext> and subsequently synced to all other nodes in the cluster, so you don't lose those changes. For my use case (sync files between home, on the go laptop, and phone, not doing simultaneous edits of the same file), which I would argue is fairly standard, I've never encountered a sync conflict.

-Syncthing is not an app for backup (see this link, section 2.2.17). Duple is; in fact backup is one of our main features.

The linked section states that while Syncthing offers file versions, they encourage the use of dedicated offline backup tools as all changes made to files are propagated across the peers. Duple interprets that as 'not backup' but in its own FAQ under question "Does it do more than file sync?" on https://doc.duple.io/faq/#technical-faqs it interprets similar functionality as 'backup' with the qualifier "data at least in 2 places". Syncthing offers similar functionality, it just doesn't try to make claims that it is a suitable replacement for dedicated offline backup software.

-Then, similar to the likes of Nextcloud, Syncthing also includes a complicated installation/setup that isn't aimed at everyone and requires some IT expertise. One of the main points of Duple is to provide an out of the box tool that anyone can use, with super easy set up.

What IT expertise is required to setup Syncthing? You copy a string from one machine and input it into another and vice versa. Then you check a box in a GUI that says 'share this directory with the other device'. Granted, that is not as common / familiar an action as logging into a form, but you're hardly hand editing any config files. However, the Syncthing foundation is working on a UI refresh that IIRC has a start-up wizard aimed at first-time users to guide them through the process of adding device and folder IDs to handshake.

-It is also worth noting that Syncthing is not planning on supporting iOS platforms.

This is Syncthing's second biggest weakness / barrier to fully mainstream adoption among techies, right behind not being able to have untrusted peers who sync data but do not have encryption keys to read it locally. According to https://docs.syncthing.net/users/faq.html#why-is-there-no-io..., this is fair. Duple's own Android and iOS mobile apps are 'coming soon' and Syncthing offers Android apps today. However, there is an iOS app named fsync which offers(ed?) Syncthing integration.