Quick comparison between AIR and Gears:

• Gears will have to be downloaded, for the web application use it.
  An AIR app will have to be downloaded.
• A web application that uses Gears has to ask special permission from the user before getting access to Gears’ services.
  An AIR app is an offline app. Users do not need to make special provisions for it.
• Offline applications written for Gears should merge seamlessly with the web interface once gears is downloaded.
  AIR applications and their web conterparts will be launched from different places. This is not necessarily a problem, but it’s not seamless. Gmail’s a brilliant example.
• A Gears application will not have access to the desktop OS. So task bar icons and such are out of the questions.
  An AIR application will always have richer access to the services of the OS on which it’s running.

So all in all I’d say use AIR if your focus is a rich desktop experience, but write a web app with Gears support if you’re keen on a seamless experience going on and off the web.

Some links about Gears:

• Google search for google gears mobile support
• Opera extends Gears support - Interesting. Mobile browsers are close to my heart.
• Google Gears Goes Mobile
• Zoho Writer Extends Mobile Support, Adds Offline Capability for Windows Mobile Using Google Gears

I’m not biased towards Gears. I just found these links while researching the viability of using it for writing web apps that can go offline.