IIRC, Syncthing is decentralized P2P and synchronizes standard tree(ish) filesystems (like most similar projects). Perkeep does away with the filesystem and instead uses a blob-based merkle-tree-like system with good properties for very long term archival (and as a side effect, can also use P2P and multiple encrypted cloud backups easily). So Syncthing can copy preexisting file structures and fits in usual workflows, but Perkeep has some data storage benefits. I don't know how their performance compares.