Bitcoin is generally considered more secure than Ethereum due to its robust Proof-of-Work (PoW) consensus mechanism, which has been tested over time. Bitcoin’s security is strengthened by its high hash rate, meaning more computational power is required to attack the network. This makes a 51% attack on Bitcoin highly unlikely due to its decentralization and network strength.

Ethereum, initially using PoW, transitioned to Proof-of-Stake (PoS) with Ethereum 2.0. While PoS improves scalability and energy efficiency, some argue it is less secure because validators, rather than miners, secure the network. However, Ethereum's slashing mechanism penalizes malicious validators, enhancing security.

Bitcoin’s simplicity also contributes to its security. It primarily serves as a store of value and payment system, minimizing vulnerabilities from smart contract bugs. Ethereum, being more complex with dApps and smart contracts, has faced security issues such as hacks and exploits in DeFi applications.

Overall, Bitcoin’s conservative approach and decentralized mining network make it the most secure blockchain for financial transactions. Ethereum, while secure, faces evolving risks due to its programmability. Both blockchains have strong security models, but Bitcoin’s longevity and resistance to attacks give it an edge in security.