Hi anaja,
You’re very welcome … and I’m glad you found us 
The short answer would be careful and objective testing on local mattresses that use the materials you are considering. You may also want to test latex / innerspring hybrids if they are available locally. This is by far the best way to find out your preferences in terms of firmness levels and the type of latex you prefer. Other than this you would be going by the “averages” of other people based on more detailed conversations with online retailers or manufacturers (see mattress firmness/comfort levels in post #2 here).
Just in case you haven’t read it yet … the tutorial post here has the basic information and the steps I would follow regardless of the type of mattress I was considering and also has links to testing guidelines that will help you test mattresses more carefully and objectively.
Phoenix