Functionally they are rather similar in what they're aiming for. Architecturally there are some differences.
OpenFreeMap uses the MapTiles format [1] which is an open source format for vector tiles that does require the attribution of the OpenMapTiles page for every map generated from it (CC-BY license). Versatiles uses the Shortbread format instead [2] which is published under a CC0 license. Instead of the SQLite based mbtiles format they developed their own container format (and a converter).
I've only started tinkering with this project a little bit cause I found it interesting after watching a CCC talk with it [3]
OpenFreeMap uses the MapTiles format [1] which is an open source format for vector tiles that does require the attribution of the OpenMapTiles page for every map generated from it (CC-BY license). Versatiles uses the Shortbread format instead [2] which is published under a CC0 license. Instead of the SQLite based mbtiles format they developed their own container format (and a converter).
I've only started tinkering with this project a little bit cause I found it interesting after watching a CCC talk with it [3]
[1] https://openmaptiles.org/
[2] https://shortbread-tiles.org/
[3] German: https://youtu.be/8A51WkJ5S8I