If you haven't read the Building Science articles on moisture management, I highly recommend reading up on that site.
Here's a good article on vapor barriers:
What's best for you will depend on where you live and what your local climate conditions are year-round.
If you live in an area where winters get cold and you plan on heating the garage, then you will likely end up with moisture problems on the inside. Warmer. more humid air meeting the cold studs will cause condensation and mold.
They KEY INSIGHT from Building Science is what I call the "golden sequence":
1) Water
2) Air
3) Vapor
That's in order of importance. If you don't keep out water, there's no point in worrying about air or vapor. Assuming you have a dry building, then you have to worry about air sealing, and if you don't control the air, there's no point in trying to control the vapor.
If your garage is watertight and well sealed against air (this is not that hard to test), then you get to worry about vapor control to prevent condensation and mold.
What is your local climate?