I'd go with 42". More separation between cabs and ceiling and easier to clean.

this is 42" with a 9' ceiling. i think you might consider stacked cabinets and not 48" ones.

Here's a kitchen we completed with 9' ceilings and we used 42" upper cabinets with a large crown moulding. The homeowners still prefered to have space above the cabinets rather than having them go all the way to the ceiling. Even with tall ceiling, 48" cabinets could make the cabinets feel top heavy. If you want them taller, stacking would be a better option.

An unbroken door height of 48 will loom over lower cabinetry, I'd investigate a beautiful detail to visually "lower" the ceiling a bit ! .....beams, coffering, something lovely and fitting the rest of your home, and use 42 inch uppers. Or stack glass doored cabinets over 42 inch solid doored cabs.

I would stack the cabinets to the ceiling with the ones on top glass with lighting and display nice things you don’t use every day.

You can emphasize the ceiling height without all the cabinets going full height which will probably overwhelm the room. Have the cabinet over the refrigerator or pantry or wall oven go all the way, but leave the others lower.

Do an overhang, ala Frank Lloyd Wright.

Try an interesting shape extended up.

See all attached photos.