Hi wannabedgood,
That’s quite a “stable” of mattresses you have 
Premature foam softening and sagging is not unusual at all with major brand mattresses. The major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (regardless of how they may feel in a showroom) along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
[quote]thats when I made the switch to Tempurpedic. At first the Tempurpedic is really comfortable, the foams really contour to your body and has a nice comfort level; but after a few months when the weather warms up the bed suddenly becomes too hot;
I also started to feel the bed doesn’t have enough support (too much “sink-in” feel) and especially when I had muscle sore and lower back pain. [/quote]
Although it would generally be a minority of people … it’s also not unusual that some people sleep hot on memory foam mattresses. The lack of support may be from choosing a model that wasn’t the best “match” for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your Personal Preferences).
As you discovered there is no such thing as a mattress that is “universally comfortable” and of course that’s just marketing vs reality. You can read more about what I call “simplified choice” mattresses in post #1 here in the simplified choice topic and there is more specific information about Casper and many of the other simplified choice mattresses in post #2 in the same topic.
You can see my thoughts about airbeds in general in this article. While any mattress can be a good match for a specific person because each person’s needs and preferences or the criteria that are most important to them can be very different … in general terms I would tend to avoid them unless there is a very compelling reason that an airbed would be a better choice for you in “real life” (outside of the many “marketing stories” that you will hear about them) than the many other options or types of mattresses that are available to you.
If you are committed to an airbed and you are convinced that there are no other types of mattresses that will meet your criteria then there are some other airbed options that are listed in post #3 here that may be much better quality/value choices than Sleep Number/Select Comfort.
[quote]For me mattress shopping is just straight up painful; too many options and I couldn’t trust the words of the salesperson. So I was pretty happy when I can buy from Casper, but it turns out it wasn’t a pleasant experience after all. It just seems even after I tried the mattress in store it’ll be a totally different feel when its in my home few months later; and my preference to sleep on the side or on the back changes periodically as well (STRANGE ME I KNOW).
I really LOVE the concept of sleep number, but I just cant find the right comfort I want within the settings. Wondering if any of you out there has the similar experience? Do you guys have any good advice for pregnant woman? Don’t think changing a mattress for 9 months then switch back is a smart decision. [/quote]
While I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
I’m not sure what you’ve read since you found the site but just in case you haven’t read it yet … the first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you.
Given that your needs and preferences seem to change from time to time it may be worth considering a component mattress where you can choose the individual layers and components of the mattress. It’s unlikely that you would need to change the deeper layers of the mattress but you could keep a firmer or softer comfort layer on hand so you can change it as you need need to.
A mattress with one or more toppers would serve a similar function and if you need to make changes to the mattress you could just add a softer topper to the mattress or change the topper for a softer or firmer version.
Phoenix