I use a wing but, for buoyancy there's not a lot of difference between a wing and a bcd until you start carrying a lot of gas. The concept is to be neutrally buoyant at 3m with an empty tank. Extra gas needs to be added to compensate for the weight of gas you start your dive with (about 2kg of air in a full 12??) and the compression of a neoprene suit (drysuit or wetsuit) at depth.
I use a membrane drysuit and find that I can adjust my buoyancy when using a single cylinder quite comfortably just adjusting the amount of gas in the drysuit, other people use a constant volume drysuit and adjust with a bit of gas in the wing/bcd. Either way is suitable as long as it suits you.
The point is there aint a lot of gas in your wing/bcd even if you are using it as your sole method of buoyancy adjustment. With me, there is no gas in my wing unless I'm sitting on the surface. If you still have gas in your wing/bcd with an empty tank at 3m then you are overweighted.
Trim is a different thing completely. This depends on WHERE the weight is situated. Moving your tank up and down in the straps has a large effect. Also, think about where your weights are situated. For me, I have weights in a pouch by my belly and some strapped to my tank. This is just a personal thing, the important thing is that I know exactly where to position my tank and weights to get the trim I want and I make sure everything is in the same place (exactly) whenever I dive.
An underwater camera is a very handy tool. How you feel your trim is in the water can be very different to how it actually is as shown on a camera.
Best of luck,