cleos create account is for developing purpose, and cleos system newaccount is for real world. And if you looking into eosio.system contract, you will find that newaccount is same as create account.

cleos create account is for boot purpose, several accounts MUST created first before deploy eosio.system contract. See more.

  eosio.bpay
  eosio.msig
  eosio.names
  eosio.ram
  eosio.ramfee
  eosio.saving
  eosio.stake
  eosio.token
  eosio.vpay

And cleos system newaccount is a normal way to create account, it is call 'action' (same like function) of eosio.system contract. And if you looking into eosio.system contract, you will find that newaccount is same as create account.

And just like Todd said, after your deploy eosio.system contract, it is not allowed to create account using cleos create account.