Hi cantona,
Just in case you haven’t read it yet … post #1 here has all the basic information, steps, and guideline that can help you make the best possible choices.
A mattress is only as good as its suitability in terms of PPP and its construction, design, and the quality of the materials inside it and comparing brands is really not an effective way to make comparisons. Any manufacturer can use any materials they choose inside their mattresses. To make meaningful quality and value comparisons you would need to know the specifics of what is inside each mattress you are comparing and the name of the manufacturer is not important. The good news is that both of the manufacturers you are looking at will usually disclose the information you need to make informed choices.
Once you are down to choices between good and good (following the steps in the tutorial post) then the “best” choice for you would depend on your personal value equation and all the objective, subjective, and intangible criteria that are most important to you.
I would also be aware that buying a mattress with the intention of adding a topper can be somewhat tricky because knowing which topper is the most suitable for the mattress you have and your specific needs and preferences can be almost as difficult as buying a mattress unless you are able to test the specific combination in person. The concept is a good one but every layer and component of a sleeping system will affect every other layer and component and choosing the right combination can be difficult if you can’t test the two together.
If you can list the layers of each specific mattress you are considering on the forum I’d be happy to help you identify any potential weak link or make some comments about their materials and design.
Phoenix