How Does a 51% Attack Differ from Selfish Mining in Terms of Network Control and Impact?

A 51% attack requires a majority of the hashrate to execute malicious actions like double-spending and censoring transactions. Selfish mining, however, only requires slightly more than 25% of the hashrate and is a revenue-maximizing strategy that doesn't necessarily involve malicious double-spends.

The 51% attack aims to rewrite history, while selfish mining aims to gain a disproportionate share of block rewards. Both exploit proof-of-work but with different objectives and required control.