First get an independent kitchen designer not a cabinet sales person who can only design from what they carry. Upper come in many heights as do the smaller ones to stack. !5/ 33 will leave even a bigger gap at the top . If you go 39/12 it will leave a 3" gap and I would want that to be as simple as possible and in the same finish as the cabinets.. If your ceilings were perfect you could do 39/15 but most ceilings are not perfect. You could raise the uppers a bit and make that gap even smaller than 3" I actually like my uppers at 20" above the counter.