Hi nancy361,
I certainly agree with this 
If the new mattress is appropriate for your body type and sleeping positions and you are “adjusting” to a mattress that has better support and alignment compared to the mattress that you are replacing then that may be worth it but if the new mattress is not appropriate for your body type and sleeping positions and the “adjustment” is because you are now sleeping out of alignment or the mattress is not appropriate for your needs and preferences … then this is not so good and could contribute to longer term issues.
No … brands are not important because it’s the materials in the mattress and it’s construction that determines how well a mattress will perform for your unique needs and preferences. You can read a bit more about “brand shopping” in this article and also the first article on the front page of the site. Any manufacturer can make a mattress out of any materials they choose. Having said that … this article may help you avoid most of the worst choices in terms of brands or manufacturers that are known to use lower quality materials (or even worse not tell you at all what is in the mattress) or charge higher prices in every budget range of their mattresses in an apples to apples comparison. In general … you are always better off by looking at mattresses that are made by smaller independent manufacturers (what the larger brands call “off brands” who don’t advertise as much if at all and which are sold either factory direct or through better sleep shops. They are the source of the best quality and value you will find. Post #10 here will give you a step by step process that will greatly improve your odds of finding the best possible quality and value in a mattress and finding the manufacturers and retailers in your area that have the knowledge and experience to help you make the most suitable choices no matter what your budget.
Some of the manufacturers and retailers that are among the best quality and value in the country have been invited to become members here (you can see them here) but there are many others that exist across the country that are not members yet who have been making exceptionally high quality mattresses with great value for generations but are often lost in the “noise” of advertising by major brands and mass market outlets.
The short version is mostly good quality (with a few exceptions like the Simplicity line) but very poor value. There are many manufacturers who make mattresses with similar or better quality materials that are an “apples to apples” comparison that sell for significantly less. They are among the major brands I would avoid. You can read more about my thoughts about Tempurpedic in this thread and in post #10 here.
The balance between the contradictory demands of “comfort” (pressure relief) and support (alignment) is of course the main challenge in choosing a mattress. The help and guidance of a salesperson with integrity, knowledge, and experience can be a huge help in making good choices. This is why it is usually more important to find the best outlets in your area before you start trying to find the best mattress. The goal is to just learn enough of the basics (the overviews are good for this) so you can ask better questions and know who is giving you good information and can help you find the experts rather than needing to become one yourself (although even reading the overviews will often give you more knowledge than many people who sell mattresses).
There are several good options in terms of couples that have different needs and preferences. One is as you suggested (two twin XL mattresses making up a King size), one is the many mattressses that are available that have “split” layers (where each side is constructed differently), and one is certain designs that because of how they are layered can accommodate different weights and shapes on the same mattress. Twin mattresses would actually lower the possibility of “roll together” not increase it because each side would act independently of the other and the heavier person wouldn’t be creating an impression that the other one may tend to roll into.
Hope this helps
Phoenix