IMO both can and will co-exist together.

Cardano seems rather promising and with smart contracts finally launching in Aug/Sept. I think it will really start to blow up once Goguen (smart contracts) era is completed. I hope to see a bunch of DeFi grow on it.

DOT is also another great one as it's ran by Gavin Wood.

Both projects and their co-founders being ex co-founders of Ethereum and then left over differences about the direction Ethereum should take.

Staking with Cardano yields 4-6% and staking DOT can yield 12%.

I'm long on both but always down to hear others opinions.