How can I set a preferred contact method in Messages
As of 10.8.2 the only way to do that is to keep that particular conversation open all the time.

Another workaround is to copy the preferred google talk info to another contact name with just that. "Jane googletalk" so you know which is which.

On a conceptual level i don't think they will fix it. It's the same with multiple phone numbers... you have to select which one each time you call... you can't have a preferred number, UNLESS you Favorite that number.

Alas there there is no favorite option in messages for mac.