Which method of framing a soffit around rectangular ductwork maximizes ceiling height?
I usually use wood for everything.  This is how I would put it together with 2x4s.  Personally, I think 2x4s are overkill for this, but that's just me.  I suggest using screws as your fasteners, and with screws your "bottom plate" is probably sound.  Screws will be fine in tension.  If you are using nails, I'd stick to the sketch so that your nails are loaded in shear.

The main code issue you are going to be dealing with here is fireblocking.  You have this open plenum that connects all the joist cavities which allows fire to spread quickly across the joists.  You'll want to stuff insulation in areas that will prevent airflow from one joist (and wall) cavity to another.