Based on all the feedback I may make this the default for 0.3, or at least show the option when gContactSync is used for the first time. Usually changing default preferences is frowned upon, so I may just do the latter and try to make the option visible and obvious to anyone who doesn't like the new phone number types.
While I would normally agree with you about not changing a default setting like this, in this case I think changing the default is the sensible thing to do.
Your extension is a Thunderbird extension, not a GMail extension. Many people - like me (can you tell?)

will only be using it as a way to backup/share their TB contacts between computers and their phone(s), and will rarely if ever even log into their gmail account through the web, so they don't *care* what Gmail sees or doesn't see.
So, I'd do the opposite - upon first run after the extension upgrade, make the new default 'visible and obvious', with a simple button click being all that is necessary to revert to the old/original behavior.
Or at least that's my .02 clad coins worth...
