Depends if you are hot or cold, I've found that Gardiners are great for cold, they really can't be faulted, however with hot water the hoses blow up too much, but I believe they are bringing out some purpose built hot hose which if it's as good as their regular hose will be perfect. Why not go the whole hog and get 6mm, from 12.5 it will be like carrying nothing.
Simon.