Google Auth has the added advantage of not having to be online to authenticate. The Google Auth mechanism I elected to use is deterministic, and therefore you can either pre-generate your keys and keep them with you, they are only good once. There's no call backs to any Google Servers or any notification to Google or anything to do with google beyond using their authenticator app on your mobile device. Definitely a plus.
They do this now? I coulda swore the last time I checked that they only supported time based mechanisms (OATH-TOTP).