But they've different purposes, ADA is the monetary transactions and support for smart contracts while ETH stand-alone with smart contract support which is only a single layer of managing the smart contract and the dApp. This is many, many projects are preferred to use ETH because it focused on one purpose of decentralization.
How many coins that competing with ETH or considering an ETH killer? But none of them has succeeded.
My vote is still on ETH but ADA is also still useful.
it will be a long way to go before another network will surpass ETH network. that is true, how many networks have been created to kill ETH but none of them so far has attained the same popularity of ETH. i believe the advantage of ETH is that it is backed by a known dev, Buterin, who is still sincere in developing its network to serve better the whole community. maybe, if his dev team will stop innovating, that's when a new network will overcome ETH's popularity.