I have 2 options for a new floor:
9mm plywood on joists, with tongue and groove floorboards on top 19mm thick
28mm tongue and groove floorboards on joists (no plywood subfloor)
Which is the better option?
If there is another space below, then you want the subfloor for no other reason that to prevent dust, air, sound and light from leaking between the floors.
I would want the T&G on the bottom and use the thin stuff as an underlayment. If you put the T&G on top your finished floor will not look as good. I have used T&G plywood with nice results but I normally put a thin underlayment on top and make sure the seams are off set for the best results.
I've been on a lot of decks that are made from 1.25" (31 mm) softwood planks, not grooved, and they aren't bouncy. I suspect that using 28mm T&G boards (stress is spread from one board to the next one) would be sufficient by itself.
The 9 + 19 may work if you glued the finish floor to the subfloor in addition to fastening at the joists.
Is the difference in price between 19 mm and 28mm enough to compensate for the cost of the plywood and the aggravation of putting down another layer?