Two main differences: chat and membership.
CHAT
PeerTube's live streaming was originally launched without chat and introduced later as an optional plugin later. PeerTube is mainly meant as a YouTube alternative, livestreaming is a side thing.
OwnCast was built as a Twitch alternative and the built-in chat is much better, with options for verifying your identity.
MEMBERSHIP
PeerTube allows other people to join a server, OwnCast is currently just for self-hosting by individuals.