The Zilliqa idea started from overcoming the limitations in scaling up existing protocols of extensively used Blockchain platform like Bitcoin or Ethereum network. The developers behind Zilliqa believe that the problems lie in the design of the consensus and network protocols of those. That is why merely re-engineering may scale up to some extent but not drastically as required to support thousands of transactions / sec and reach its full potential. In my opinion Zilliqa is better than cardano.