The connector is the least interesting part of a security key and the part most likely to make it useless to you. A key that will not physically go into the machine you sign in on is an expensive keyring ornament.
USB-A
The older, larger rectangular port. Still everywhere: desktops, docks, monitors, meeting room hardware, and a good deal of the equipment sitting in Australian offices right now.
Buy USB-A if your primary machine is a desktop or an older laptop, or if you work across a fleet where USB-A is the common denominator. The YubiKey 5 NFC is the USB-A model.
USB-C
The small reversible port. Standard on laptops sold in the last several years, on every current MacBook, on most Android phones, and on recent iPhones.
Buy USB-C if your primary machine is a modern laptop. That is the 5C NFC if you want a key that also taps against a phone, or the 5C Nano if you want one that lives permanently in the port.
USB-C is the safer bet for the future, but "future" is not the test. The test is the machine you sign in on under pressure.
Lightning
Apple's older connector, used by iPhones before the switch to USB-C. Yubico makes the 5Ci for this, with Lightning on one end and USB-C on the other.
Here is the honest position: this is now a legacy-device product. Recent iPhones use USB-C, which means the 5Ci solves a problem that is steadily disappearing. If you are buying today for a current iPhone, you almost certainly do not want it.
There is one case where it still makes sense. If you have older iPhones in service, need to plug in rather than tap, and want a single key that also works with USB-C laptops, the 5Ci does that specific job and nothing else does it as neatly.
We do not stock it. If that is your situation, buy it from someone who does rather than buying the wrong thing from us.
When NFC makes the connector question smaller
NFC lets you tap a key against a phone instead of plugging it in. Both iPhone and Android support it for security key sign-in.
That matters here because it removes the phone from the connector decision entirely. With an NFC model, you choose the connector for your computer and let tapping handle the phone. This is why the 5C NFC suits so many people. USB-C for the laptop, NFC for the phone, one key.
What NFC does not do is help with computers. Almost no laptop or desktop has an NFC reader, so on a computer you are plugging in regardless.
Adapters
They work. USB-C to USB-A adapters are cheap and reliable.
They are also small, easy to lose, and never present at the moment you need them. Every adapter is a second object that has to be in the same place as the first one. For a single personal key that is a manageable annoyance. Across a team it becomes a recurring support ticket, and the money saved does not cover the time.
Buy the connector you have.