For several contacts in my address book, I have both a Google Talk and an AIM handle. Messages on OS X (10.9.2) seems to always default to the AIM-based account for these people when they're signed into both, but I'd like to change that to use Google Talk by default. I can't find any way in the contact's Info window to change this.
Figured it out: under Messages' Preferences: Accounts options, the sort order of the accounts controls the ordering when sending a message. Dragging and dropping my Google Talk account above AIM fixed it (though this applies to all contacts, I can't find a way to change it on a per-person basis.)