1. Purpose and Functionality: Bitcoin was created as a digital currency and payment system, aimed at providing an alternative to traditional fiat currencies. It serves as a store of value and a medium of exchange. Ethereum, on the other hand, goes beyond being just a digital currency. It is a decentralized platform that enables the creation and execution of smart contracts and decentralized applications (dapps).
2. Blockchain Technology: Both Bitcoin and Ethereum utilize blockchain technology, but they have different ways of implementing it. Bitcoin uses a simpler blockchain design focused on maintaining a secure, immutable transaction ledger. Ethereum, on the other hand, employs a more complex blockchain structure called Ethereum Virtual Machine (EVM), which allows for the execution of smart contracts.
3. Consensus Mechanism: Bitcoin uses a proof-of-work (PoW) consensus mechanism, where miners compete to solve complex mathematical problems to validate transactions and secure the network. Ethereum initially used the same PoW model, but it is in the process of transitioning to a proof-of-stake (PoS) mechanism called Ethereum 2.0. PoS relies on validators who hold and "stake" their cryptocurrency to validate transactions and maintain network security.
4. Scalability: Bitcoin has faced scalability issues, with high transaction fees and slow confirmation times during periods of high demand. Ethereum has also experienced similar challenges. However, Ethereum has been actively working on scalability solutions like layer 2 protocols (such as the Lightning Network) and Ethereum 2.0, which aims to increase transaction throughput.
5. Use Cases: Bitcoin's primary use case is as a digital currency, serving as a decentralized form of money and a hedge against inflation. It is primarily used for transactions and as a store of value. Ethereum, with its smart contract capabilities, has a broader range of use cases. It enables developers to create decentralized applications, launch initial coin offerings (ICOs), and build decentralized finance (DeFi) protocols, among others.
6. Market Share and Adoption: Bitcoin has been the dominant player in the cryptocurrency space since its invention in 2009. It has a larger market capitalization and has gained more widespread adoption as a digital currency. Ethereum, however, has gained popularity due to its versatility and development platforms, attracting numerous projects, developers, and investors to its ecosystem.

In conclusion, while both Bitcoin and Ethereum are prominent cryptocurrencies built on blockchain technology, they have differing design philosophies, functionality, use cases, and approaches to consensus. Bitcoin primarily focuses on being a digital currency and store of value, while Ethereum extends its capabilities to include smart contract functionality and the creation of decentralized applications. The choice between the two depends on the specific goals and requirements of the user.