Should I assume the zip hoodie fits the same as the non-zip variety?

First, yay!!! @ this being reprinted..and on Christmas?! Merry to me. Anywho.. I normally wear a size Large in Girly..and it's a bit room-y, but Medium is toooo snug for my taste. Now I have never bought a hoodie from Threadless before and I was wondering what size I would be? I heard someone mention before to order the same as your t-shirt size, but that seems far too big. According to the measurements, I think I'd need an XS or S hoodie..anyone care to lend advice on sizing? Particularly Girlies who wear Large? =) Thanks!!