The YubiKey Bio adds a fingerprint sensor to the key. The 5 Series asks for a PIN instead. Both are FIDO2, both are phishing-resistant, and the choice between them is about convenience and context rather than about strength.
What the second factor is actually doing
A security key on its own proves possession. You have the key.
For anything sensitive, that is not enough by itself, because a key someone picks up off a desk is a key that works. So FIDO2 supports user verification, a second check that the person holding the key is the person it belongs to. That check is either something you know, a PIN, or something you are, a fingerprint.
Both satisfy the same requirement. When a service or a Conditional Access policy demands user verification, either one meets it.
What biometrics genuinely buy you
Speed, and the absence of a memorised secret.
Touching a sensor is faster than typing a PIN, and over dozens of sign-ins a week that adds up in a way people notice. There is no PIN to forget after a holiday, no PIN written on a sticky note, and no PIN shared between two people who share a workstation.
For an executive who signs in constantly and resents every extra second, or for a workforce that will quietly undermine anything inconvenient, that is a real argument.
Where a PIN is the better fit
Gloves. Cold hands. Wet hands. Anyone in a kitchen, a workshop, a clinical setting or a cool room. Fingerprint sensors are unreliable in exactly the environments where people are least able to stop and try again.
Shared and role-based keys are the other case. A biometric key is enrolled to specific fingerprints, which makes it personal by design. That is a feature for an individual and an obstacle for a key that belongs to a role, sits in a safe, or gets handed to whoever is on shift. A PIN can be handed over. A fingerprint cannot.
Price is the third consideration. The Bio series sits above the 5 Series, and the money spent making one key faster is money not spent on the second key every person needs.
The protocol difference nobody mentions
The Bio series is FIDO-focused. The 5 Series speaks FIDO2 plus smart card, OpenPGP, stored OATH codes and Yubico OTP.
If you need any of those, the comparison is over before biometrics enter into it. Check what your accounts require before you weigh convenience.
What we would tell most buyers
For most people and most organisations, a 5 Series with a PIN is the right call. It is cheaper, it works in every environment, it can be reassigned, and it covers more protocols. The PIN is a small tax paid a few times a day.
The Bio earns its place with a specific group. High-volume sign-ins, personal keys that will never be shared, and clean dry hands.