Google has simplified its range of Titan security keys by dropping its Bluetooth Titan Security Key and making greater use of NFC.
Moving forward, NFC will now be offered on both the USB-A and USB-C keys, which means that most users will be able to streamline their keys down to a single unit.
If you have an older system with USB-A ports, Google recommends that you buy a USB-A + NFC security key, which should work with most smartphones and tablets. This is the key recommended for iPad users who have a Lightning port on the iPad (they will also need an Apple Lightning adapter).
If you have a more modern system that makes use of USB-C, then the key for you will be the USB-C + NFC security key.
Bluetooth Titan Security Keys will continue to work, and warranties will continue to the honored by Google.
The USB-A+NFC security key, which comes with a USB-A to USB-C adapter, costs $30, while the USB-C+NFC security key costs $35. Both are available from the Google Store.
A good alternative to Google's Titan security keys are the YubiKey line, and come in a wide variety of options.