Hi jirika,
I’m not familiar with the softness or “feel” of the different mLily mattresses but your own testing in the store should give you a good idea of which of their models feel softer to you than the one you have.
mLily is a Chinese brand but they are CertiPur certified and make some relatively good quality/value mattresses as long as you are comfortable with buying a mattress that is made in China and any extra risk that can come from buying a mattress that may be compressed for longer periods of time in shipping and storage (see post #6 here).
Like any mattress though … outside of how well it matches your specific needs and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) … a mattress is only as good as the quality of the materials and components inside it and I would always make sure you know the quality/density of all the layers in any mattress you purchase so you can identify any weak links and make more meaningful comparisons to other mattresses … regardless of the name of the manufacturer.
A forum search on mLily (you can just click the link) will also bring up more information and feedback about them as well.
Durability issues with a pillowtop would depend on the quality/density of the materials inside the body of the mattress and the pillowtop because lower quality materials will soften and sag more quickly than higher quality/density materials (such as latex) which can lead to the loss of comfort and/or support regardless of which type of mattress they are in or who makes the mattress. The “latex” pillowtop may also have other materials inside it as well but if the Chateau Collection mattress you are looking at is made by Stress-O-Pedic then they are transparent about all the materials in their mattresses (see this topic) which means that you would be able to find out all the information you need (see this article) to compare them to the guidelines in post #4 here so you can make an informed choice in terms of suitability based on your testing and in terms of quality and durability based on the materials in the mattress. There is also more about the most important parts of the “value” of a mattress purchase in post #13 here.
Phoenix