Hi toddking2,
Hi. Thanks so much for the detailed and thoughtful reply. I cannot return to APMC to test the 3" topper because I do not live in Phoenix. I visited APMC while in Phoenix briefly on business, and now I am back on the East Coast where I live, with no plans to return to Phoenix.
Well that rules out re-testing so we're left with some thoughtful analysis and a more detailed conversation with Ken.
Here are some questions I would consider when you talk with Ken that may provide some insights.
Your body type and weight distribution and differential between your waist and your shoulders.
What type of mattress have you tended to do best with in the past ... soft, medium, firm?
Do you have any history with either pressure issues or alignment issues on any mattresses?
What position to you tend to go to sleep in and what position do you tend to wake up in?
What type of mattress are you currently sleeping on and what are the reasons you are replacing it?
What specific differences did you notice when you tested the medium core vs the soft core?
These questions and talking about your answers in more detail can provide some good guidance.
The suitability of the mattress for your girlfriend may also play a role and I would let Ken know more about her body type and sleeping positions as well so to some degree you can include her in your choice.
I also wouldn't be at all concerned about the durability of either a 28 ILD or a 32 ILD core so if it was me this wouldn't be a factor in my choice.
If you do decide on split cores to accommodate both you and your girlfriend I would tend to keep it to one firmness level different because more than this may feel a little "odd" if you sleep in the middle of the mattress. The difference between a soft/medium split would be less noticeable. Did you test this in the showroom and if you did what did you feel?
If you both go to sleep and also wake up on your back and don't have any specific history of symptoms that could cause you to sleep in a less preferred position to "avoid" symptoms then it may indicate that your "primary" sleeping position is your back and I would tend to lean towards a design that prioritized this position (but of course your conversation with Ken may also bring up some information that could change this).
Unless your sleeping history indicates otherwise or you have a more athletic body type (wider than normal shoulders and upper body or a deeper lumbar curve for example) I would tend to prioritize alignment/support over pressure relief. Pressure point relief that is fairly close but just a little off is much easier to fine tune with a plush mattress pad for specific pressure point relief for example than fixing support/alignment issues and the back symptoms that can come from them.
Phoenix