Hi serene1,
This will depend to some degree on your sensitivity and where you are in the range between “princess and the pea” and “I can sleep on anything”. In general terms the upper layers (the top 3" - 6") of a mattress will contribute most of what you feel when you first lie on a mattress or when you are going to sleep at night but the deeper layers can also play a role in what you feel when you wake up in the morning (either with or without discomfort or pain in your lower back especially that can come from sleeping out of alignment).
With lighter weights that don’t sink into a mattress as deeply the deepest layers will have a less noticeable effect on what you feel or how you sleep than they would for those who are in heavier weight ranges so there would be many people in your weight range that wouldn’t notice any significant difference between a 10" mattress where the bottom 4" was firm and a mattress where the bottom 4" was medium although there would still be some people that may notice a difference … especially based on how they feel when they wake up in the morning.
If two different mattresses are very similar or the same to you then I would tend to choose the firmer of the two.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
Phoenix