KeepKey IMO has a security flaw in that passphrase encryption of your seed is not enabled by default. KeepKey can certainly enable passphrases but they have not. Management must have had a bad experience in the past losing a passphrase. To protect your Trezor against physical attack encrypting your seed with a passphrase is a good option, as long as you feel comfortable with it. If you lost your passphrase you would lose your bitcoin. However the seed would be worthless to a thief without the passphrase. Trezor gives you the option it is your choice. Because of the way Ledger uses a secure element passphrases are not required or needed. However you may miss the convenience of the screen Trezor provides. It is good to have choices.