Hi Loiroi,
The major brands (such as Sealy/Stearns & Foster, Simmons, and Serta) are generally among the least transparent in the industry and they 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 (and the major retailers that focus on them) 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).
Phoenix