SCTP
      Low level. Very configurable.
      Message-based: reliable and unreliable, ordered and unordered.
      Automatic fragmentation and reassembly of large messages.
      Encryption and peer authentication not included: use DTLS.
      Socket layer not included(*): use UDP.
      Minimum MTU is configurable.

QUIC
      High level. Less configurable.
      Channels (reliable byte streams) and datagrams (unreliable).
      Datagrams are MTU limited (no fragmentation/reassembly)
      Encryption, peer authentication and sockets handled by QUIC.
      Minimum MTU is 1228 bytes.


(*) IP-Level SCTP exists but is not supported everywhere, so it has
    to be encapsulated over UDP.

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Big Picture
    Both protocols are well-engineered, real-world tested, and fully featured. Either
    would be a significant improvement.

    SCTP is better for raw performance and customizability. SCTP was originally for
    telephony networks, where the companies control and tune both ends of the network.
    QUIC was intended as a general replacement for TCP, with ease-of-use for developers
    as a main goal.

My Current Take
    I would favor QUIC, because QUIC owns the entire network stack, so there will be
    less code to maintain, and less to configure. This does increase the chance that
    the QUIC implementation has quirks or bugs that need to be patched, but to assume
    that in advance would be premature. A deal breaker for QUIC would be if a
    significant number of users find that their networks can't handle the 1228 MTU
    requirement.

    In favor of SCTP is that it better matches the existing protocol. It has direct
    equivalents to both reliable and split packets.

    But adding message framing to a reliable byte stream is easy, allowing for
    reliable messages of any size on QUIC. And there are only two cases where Luanti
    may emit an unreliable packet that needs splitting:

    - TOCLIENT_PLAY_SOUND:
        Splits if the sound name is above 460 bytes.

    - TOCLIENT_ACTIVE_OBJECT_MESSAGES:
        Splits if the number of ActiveObjectMessages exceeds 8.

    Both of these can be adapted to not require splitting.