Hi ttran,
The first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that you will need to make the best possible choices … and know how and why to avoid the worst ones.
I would be very cautious about all of the mattresses you are considering and most importantly I would always make sure you know the quality/density of all the foam that is inside any mattress you purchase (see this article).
You can read more about the Brentwood mattresses in this topic and in this topic as well.
Night Therapy is one of the Zinus brands and you can read a little more about them in post #31 and #32 here along with post #2 here. A forum search on “Night Therapy” will also bring up more information about them as well including this post which would be a typical experience with lower density or less durable materials.
Slumber Solutions is a little better option than the first two you mentioned and they are more transparent and list the density of their layers in the Overstock description. They say they are US made but they don’t say whether the mattress is assembled in the US from Chinese foams or if the foam itself is made in the US. Their 3.5 lb memory foam is also a little lower density than the durability guidelines I would suggest using (see here) but overall they are “not bad” although there are better quality/value options available to you IMO.
Some of the manufacturers that make some lower budget memory foam or polyfoam mattresses that may be worth considering are listed in post #2 here. If a latex or latex hybrid mattress is also something that you are interested in considering then posts #3 and #4 here include some of the lower budget latex and latex hybrid mattresses that I’m aware of although these tend to be in a higher price range than less costly materials like memory foam or polyfoam. There is more about how latex compares to memory foam in post #2 here.
This is also completely normal because firmness and softness are very subjective and relative to the body type, sleeping positions, and sensitivity of the person along with what they are used to sleeping on. A mattress that feels “too firm” for one person can feel “too soft” for the next. The most important part of the “value” of a mattress is how well it matches your specific needs and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and there are only two ways to know whether a mattress is a good match for you. The first of these is your own careful and objective testing using the testing guidelines in the tutorial post which is the most reliable of all. If you can’t test a mattress in person then the most reliable way to know if a mattress is a good match for you is a more detailed conversation on the phone with a knowledgeable and experienced manufacturer or retailer that has your best interests at heart and can help you choose which of their mattresses is likely to be the most suitable for you (see mattress firmness/comfort levels in post #2 here). No matter what the cost or quality of a mattress if it’s not suitable for you to sleep on then it would have little value to you regardless of how well it works for anyone else.
If after a more detailed conversation you still aren’t confident that a particular mattress is a good match for you then the return or exchange policy and any costs involved would also be an important part of the “value” of a mattress purchase so you can use your own sleeping experience and perceptions to decide if a mattress is suitable for you (regardless of what anyone else may think about the same mattress) and if it isn’t then you would still have options to return or exchange it. There is also more about the most important parts of the “value” of a mattress purchase in post #13 here.
As Brass also mentioned … if you don’t like the feeling of sinking in or sleeping “in” a mattress then memory foam … especially in thicker layers … may not be the best choice for you and a more resilient material that has a more “on” the mattress feeling (such as polyfoam or latex) may be a more suitable choice.
The thickness of a mattress is only one of several factors that affects the firmness or softness of a mattress but it’s not the most important one (see post #4 here). Every material (such as memory foam) has firmer and softer versions and this along with the thickness of the softer comfort layers will also have a bigger effect than the thickness of the mattress itself. In other words an 8" mattress can be much softer than a 12" mattress that uses different types of materials and has a different design. Overall I wouldn’t pay nearly as much attention to the thickness of a mattress as the other factors that can affect the firmness/softness of a mattress. Once again … besides testing a mattress in person, the only way to have reliable information about the softness or firmness of a mattress relative to other mattresses that are sold by the same manufacturer or retailer would be a more detailed conversation on the phone with someone that was experienced and knowledgeable and where helping you choose a mattress that was suitable for you was more important than just selling you anything they could.
Overall I would follow the steps in the tutorial post, do some local testing to see which types of materials or mattresses you tend to prefer, and make sure you know the quality of all the materials in any mattress you are considering so you have the best possible chance of buying a mattress that is a good match for you in terms of PPP and all the other parts of your personal value equation that are most important to you.
Phoenix