The difference is that the Ethereum platform is Turing-complete. Various new functionality can be developed with relative ease. This makes the blockchain Ethereum often also called programmable blockchain. While Bitcoin has only one function that aims to facilitate peer-to-peer transactions as a digital currency.
Bitcoins is the first cryptocurrency that has been created therefore it get all the popularity and ethereum today is the next coin which is altcoin that has developers who work hard to promote their ico and become the next to bitcoins.