Totally different beasts.
As I understand, Ardor is a new 'master blockchain' that handles all the security of underlying child-blockchains. That way the child-blockchains can be pruned easily and stay small and fast. Apart from securing the child-chains Ardor won't have a lot of functionality itself. The features (compared to present NXT-blockchain: asset exchange, voting, messaging etc) will all be in the various (white-label?) childchains.
The comparison with Ethereum is that Ethereum tries to do smart contracts, where the Ardor child-chains do smart (phased) transactions, which is a lot easier (and thus more secure).