Seafile is significantly faster at syncing than Nextcloud/Owncloud, particularly if you have a lot of small files. It’s because the core is written in C and is much more efficient than Nextcloud’s PHP code. They also recently released a new “Drive Client” (https://www.seafile.com/en/help/drive_client/) that only downloads files when you need them, rather than always syncing everything. Very useful if you have a LOT of files “in the cloud” that you rarely use, but still want quick access to.
Also, Seafile focuses only on file syncing. They don’t have a whole heap of other random stuff you may not even use.