I've got a couple of different styles that I've gotten for different newfies. To be fair though, one was purchased because it was the only one that would fit on the big boy. The ones that I have mainly are ones with two straps across the back and one under the belly and one across the chest.
I don't think that it really matters much what goes over the back as long as you make clear to the dog that you are happy with them wearing it.
Our first newf would carry Ian up the snow covered hill (we created a system where she would have the sled clipped onto her backpack to drag him and the sled back up the hill
) all day long as long as we were there to tell her that she was a good girl (which she was more than we knew at the time). She would also carry groceries home in the same pack with a proud wave to her tail when we took her to the store.
It really all boils down to the importance that
YOU place on the pack in her mind. If she feels that she is helping out/providing for the family, you will get a much better response than if it is an occasional thing that is used to show off for the in-laws that you have a big dog that can carry a 12 pack of beer.
Greg