In terms of metadata privacy, NIP-17 is better than NIP-4. NIP-17 provides better protection for the sender's ID, but it does not sufficiently protect the recipient's ID. The visibility of someone continuously receiving messages is also a metadata privacy issue. For a DM (a feature subsidiary to microblogging), this might be an acceptable tradeoff, as users can receive and decrypt messages on multiple clients. However, for an application using Nostr for a chat app, privacy should be a priority.