Short version: the FIPS is for enterprise. The Security Key is for consumers. The 5 series is for enterprise or power users.
This might be more useful for you: https://www.yubico.com/products/yubikey-hardware/compare-pro...
You want the Yubikey FIPS if you're using it in a context where FIPS compliance matters, such as US government. If not (such as for personal use), then don't bother.
The Security Key series is the budget minimal "works as a FIDO authenticator". It lacks the bells and whistles that come with the 5 series. AFAIK it is not "less secure," it just lacks bells and whistles.
The 5 series works as a FIDO authenticator, but it also includes PIV capabilities (smartcards, PGP, etc.), which allow you to do stuff like this: https://developers.yubico.com/PGP/SSH_authentication/.* The PIV capabilities matter more in the context of enterprise/government applications where smartcards have been in common use as a 2nd factor for many years, which makes having an all-in-one smartcard+FIDO device very convenient.
* Caveat: I recommend against using your Yubikey for SSH private/public key authentication unless your SSH setup also requires a password. Single physical factor is worse than having a password protected key pair in your ~/.ssh folder.