Lightning is the longest-running production micropayment rail. x402 (when paired with USDC on Base) is a much newer micropayment-capable rail using EVM stablecoins. They're both genuinely useful for sub-cent AI agent calls — the choice is mostly about whether your agent already speaks Lightning or EVM.
x402 wins when you want USD-stable pricing and on-chain audit. Lightning wins when you want true sub-second latency and your stack is already Lightning-native.
Lightning is faster end-to-end, but for a typical AI agent call (sub-cent, tolerable up to ~5s) both rails feel instant.
USD-stable pricing matters most when an agent's budget is set in USD by humans. BTC-denominated pricing forces the agent to re-quote constantly.