I am planning to develop a DEX similar to Uniswap or dydx on the Cardano platform.
Do I need to use Plutus or Marlowe? Which is appropriate? Of course Plutus?
A decentralized exchange is significantly more complex than just the smart contract language and implementation details for blockchain transactions. That said you can use either.
I would humbly suggest doing a lot more research during your project planning. This should include existing Cardano DEX projects: Cardano Exchanges (DEX) projects on the Cardano(ADA) Blockchain | Prices, Charts, Team & Ratings