Keystone is an air-gapped hardware wallet with a large touchscreen and a focus on transparency. Ledger is the market leader with a certified secure element and the broadest software ecosystem.
Keystone's firmware is open source and auditable, and it signs transactions via QR codes. Ledger uses a closed secure element but publishes much of its app layer and has strong independent security research behind it.
Ledger supports over 5,500 assets and integrates deeply with third-party wallets like MetaMask and Phantom. Keystone also works with those wallets but is more commonly used as a pure signing device.
Keystone's big screen is nice for reviewing transactions. Ledger's smaller screens are fine but less readable. Ledger Live still offers the most complete single-app experience.
Keystone is a great choice for open-source enthusiasts who want air-gapping. Ledger remains the best all-rounder for most holders.