Hi Jezo,
Memory foam is not a single product and there are hundreds of different types and quality levels available (and as you discovered combinations with other materials) … each with pros and cons attached to them. You can get a sense of the different properties of different types of memory foam from post #9 here along with post #8 here.
With all of this … you are really dependent on either knowing the exact type of memory foam topper that you want (because you’ve tried it on your mattress or a similar mattress), on a money back guarantee, or on the knowledge and experience of the retailer or manufacturer that is selling it to you who can explain how their topper feels and performs relative to other types of memory foam toppers and can provide you with accurate information about its quality/density as well.
The type of memory foam topper that may work best for each person depends on their body type, their sleeping positions, their preferences, and perhaps most of all on the layering and materials of the mattress they are using it on. Thinner layers will allow you to feel more of the layers that are below them and thicker layers will provide more of the feel of the topper itself. Thinner layers are “safer” because they don’t separate you from the support layers of your mattress as much and have less chance of providing the extra pressure relief you want at the risk of worse alignment.
With toppers … I generally recommend the thinnest possible layer that will “do the job” you are looking for because this is also less risky … especially if your mattress already has thick/soft layers on top.
So the key is to make your best choice in terms of the memory foam properties you are looking for (I would identify the qualities that are most important to you) and to choose the thickness that would solve the specific reasons you are looking for a memory foam topper in the first place … be most suitable for your body type, sleeping positions, preferences, and the layering of the mattress it is going on, … and has the best combination of and tradeoff between cost and quality that you are comfortable with. A good starting point is 2" (even less in some cases depending on what you are trying to achieve) and then add thickness if you have specific reasons to do so and if your mattress/topper combination has enough “room” to add extra thickness without compromising support and alignment.
Some of the better sources for memory foam toppers are listed in post #4 here.
Phoenix