I prefer a duffle, but with long enough handles, I can sling it easily over the shoulders without an additional strap. I prefer the larger opening on a duffle versus the backpack style.
Agree, a duffel that you can hook over one or both shoulders. I used to have one that opened at the end and it was really a hassle to load and unload. We put the duffel inside our gear bags and everything we'll use in our first dives inside the duffel, then (in theory) just pull it out on our way to the first day's dives.