Duo offers a number of features above and beyond Google Authenticator:

- more flexibility in choice of authentication method (voice, SMS, soft token, hard token, Duo Push)

- more supported mobile platforms (Android, iPhone, Blackberry, WinMo, Palm WebOS, Symbian, J2ME)

- more integrations (VPNs, UNIX logins, web apps, etc)

Not to mention all the features that come along with being a fully-fledged authentication service such as account management, fraud alerting, logging, etc. Feel free to give it a shot and decide for yourself. :-)

Disclaimer: if it's not obvious from my gushing, I'm affiliated with Duo. ;-)

Well, Dug Song of Duo Security points out that you can't use Google Authenticator and public key authentication in conjunction with each other over SSH at the bottom of this howto: http://www.mnxsolutions.com/security/two-factor-ssh-with-goo...