Short answer
Dropbox Transfer is useful for teams already working inside Dropbox. ZipPigeon is narrower: it is built for sensitive handoffs where the sender wants encryption before upload, encrypted storage, recipient access, and less dependence on broad shared-folder permissions.
When Dropbox Transfer may fit
Dropbox Transfer can fit teams that already manage files in Dropbox and want transfer features inside that ecosystem. It may also be simpler when the recipient relationship is not especially sensitive.
When ZipPigeon may fit
ZipPigeon may fit when the transfer is a confidential external handoff, the sender wants a delivery workflow separate from long-term storage, and the security explanation needs to be clear about file contents, keys, metadata, and recipient access.