Difference between createmultisig and addmultisigaddress?
createmultisig creates a P2SH multisig address for use in raw transactions.  It outputs the redeem script because you'll need that to spend any payments sent to the P2SH address.
addmultisigaddress adds a P2SH multisig address to your Bitcoin Core wallet, allowing you to track and spend payments received by that address.  It doesn't output the redeem script because Bitcoin Core stores that for you.  (You can retrieve it later with validateaddress.)