As others have said, it really doesn't matter. Personally, my Yorkie has never worn a collar. I don't see the point as I would never attach anything to it. She has her tag on her harness.
My Chi wears a harness as he's a bit of a Houdini we used a collar and lead on him and he slipped it a few times so for safety's sake we stick to the harness so he can't escape. He can be snappy at the vets so we always pop him his muzzle on but he's found if he puts his head down he can paw it off, should've called him Houdini!!!
Thanks for all the advice. I've ended up buying two as the very nice man in the shop said he would happily take either one back if they don't fit. I haven't gone for big fleecy ones, just padded normal ones.