This is sort of like buying a couch. It's best to find a local foam store (they really do exist) and sit on it. I mean sit on the foam...

I cut and pasted this response I gave to a post from a while back:

I found a store near my home (Southern
Calif.) that has many choices and will custom cut the foam to the size
and thickness you want. I wanted something very firm so I ordered the
"Q41" (2.8 density, 41 firmness). Prior to inserting the foam into the
fabric cover, I sprayed the foam with a special adhesive and wrapped it
with 1" thick Dacron batting.

To get an online price quote and to learn about various foam types...
open the A-1 link below, pick on the Foam link at the top. Next, pick
on the square shape. Then, it will ask you to select
1. dimensions you want,
2. how the foam will be used,
3. select foam type.... just below #3 in small print, you will pick on
"Click here to learn more about choosing the right foam for you". Here
you will learn about various cushion/foam firmness and density and the
numerical system used to rate a foam's firmness.
It will also have a chart for various foam types and typical uses including mattresses & breakfast nook applications.

I noticed that they can send you 10" x 10" samples. What you pay for the samples is applicable to any future order.
In my case, I was able to feel all the samples in the store.

-Laurie

Here is a link that might be useful: A-1 Foam and Fabrics

Thank you very much!