Whether upper cabinets are 30“, 36”, or 39” high, most women of average height will need a step stool to get to upper shelves, regardless. Therefore the idea that you will need a step stool should not be a factor in your decision.

A 30“ high cabinet is too short, both in what it will hold, and in how it will look with a 9’ ceiling.

I don’t understand why your only two choices are 30“ or 40”.

You need to decide whether you want your cabinets to meet the ceiling. If you do not, then get 42” tall cabinets for your kitchen with 9’ ceiling. If you do, then you will need to spend the money for stacked cabinets. It is a high-end look, and is more costly. The stack looks best in the proportions of either 30” cabinet + 18” cabinet above it + 6” crown molding. Or, 33” cabinet + 18” cabinet above it + 3” crown molding.

Most people would rather have more closed storage, even if the top shelves just hold things used rarely, than have space gathering dust on top.

I'm using the IKEA kitchen planner for my cabinets and they only offer 30" and 40" uppers. My plan was to make a display shelf on top and then put in crown molding that will tie it in. Kind of like this:

Except for the cost of cabinets to meet a 9’ ceiling @acm. If the OP is looking at 30” tall cabinets, it could be her budget cannot accommodate cabinets that meet the ceiling.

40” is a nonstandard size in the industry. One of the many IKEA quirks that don’t play well with standardized American SAE measurements.

With a 108” ceiling, you can do standard 30” cabinets at the top height of 84”, and then 18” ones over that, to reach 102”. Then 6” of compound crown molding to the ceiling.

Or just do standard height 42” cabinets, with a 3”-4” of crown on top, and have the room crown at the ceiling above it.

@User, I like your first idea. So genius!

@Lynn - glad you like the idea. I wrote it several posts up.

It depends on how much storage space you need and if you like the look of cabinets meeting the ceiling. If you have items that you never use but you're still not getting rid of them, go with 40”. Stacked cabinets may be best solution.