This is a continuation of my answer to Tropics Diver's question as to why I would prefer a jacket to a BP/Wing for singles...
Since this thread started with a question about a Pro QD, I'll use a dive where I'd choose that one as an example.
The mission: take a class of new divers on their first Gulf dive.
The boat: 85' converted crew boat
The weather: Good - winds WSW 10-15, sunny
The water conditions: Seas 3 feet, close together, light current, visibility 30', surface temp 75, bottom 65.
The site: Artificial reef consisting of a sunken Fleet Tug, 180' long, the wheelhouse at 40', 80' to the sand.
I have surface duty getting the students oriented after their giant stride, calmed if necessary and headed in the right direction, and headed down.
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The overriding issue on this dive is student safety. Although they have had four open water dives and successfully demonstrated all skills required to be certified in the relatively benign conditions of the Springs, Lake or Bay, this is their first trip dealing with a big rocking boat, salt water, reasonably intimidating (for a newbie) seas and current, and in many cases nausea. The surface of the sea is where things happen, so surface characteristics of my rig are very important. I may need stability, lots of lift (neutral +20), and leverage. I also want to use a single 80 because that's the lightest tank I have available that gives me enough gas for the dive after we do get off the surface - and I want a light rig on the surface for mobility and quickness.
Because of the water temperature I will select a 5 mil wetsuit rather than my 7 mil neoprene drysuit so Ill only have to carry 18 instead of 28 pounds of lead. 8 of that will go in the trim pockets of the QD and the other 10 in the ditchable pouches.
If I were to use a backplate and wing for a single, Id use my Fred T heavy plate (which would keep the ditchable weight in my weightbelt the same 10 pounds as would be in the ditchable pouches of the QD, so we can just compare lift) and my Dive-Rite travel wing. Lets compare
Raw lift: QD 24 BP/TW 30 so both have enough, with the advantage to the Wing.
Stability: Floating on the surface, the QD allows a near vertical stable posture; the BP is stable on my back or belly. I want vertical so I can comfortably get nose-to-nose with a student.
Leverage: By this I mean the ability to apply a solid, reassuring steady lift to a student by grabbing their BC while talking to them face-to-face. This is easy to do in the QD, but very difficult in a BP/Wing, because when in this vertical position with a full wing the BP/Wing tends to pitch you forward, when you need lift out front to help you pitch back. For this reason alone I rule out the backplate/wing for this dive.
Another thing I dont like about backplates with singles, whether they have a STA or not (the Fred T doesnt need one) is that they position the tank valve further aft, away from the head, and therefore in a more difficult position to reach than a jacket does. This isnt a problem with doubles as the center channel of the BP is between the tanks, but with a single sitting on the ridge of the channel it pushes the tank away. If I cant reach and manipulate my tank valve(s) easily Im not a happy camper, and that half to three quarter inch displacement I find very irritating.
Under water I personally find that the Pro-QD has excellent trim and balance; I have no trouble at all maintaining an effortless horizontal position and I find it a little easier to maintain positions other than horizontal with the Pro QD if I want to very useful in photography.
For travel in the tropics I take a Seaquest Explorer. It packs smaller and lighter than even the AL BP/TW combination, and is far superior for maneuvering into strange positions to take pictures.
I could go on, but that should give some idea of why I find the jacket more comfy with a single.
Rick
