Hi fridadc,
You’re welcome! Posting links usually isn’t an issue unless it’s promotional, but I appreciate your discretion and concern.
Regarding your first three choices, the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable 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 (along with the major retailers that focus on them as well) 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).
As for your Room and Board Encased Coil Firm mattress (made by Restwell), it uses a pocketed spring unit for support with a foam-encasement to help with extra edge reinforcement. On top of this is 2" of soft convoluted polyfoam. There is no density listed, and you’d need to find that out in order to make an educated choice about the durability of this product. The quilt panel also has .5" of polyfoam, and you’d want to learn the density of that material as well.
I’m glad that Room and Board has been providing you good service with your comfort exchange, but you’d want to find out the density of those materials to determine if this mattress would be something for you to consider keeping if the comfort meets with you needs. While comfort is easier to assess in a showroom, you of course can’t feel quality or durability, so that is the reason for the emphasis on the specifications of the componentry within the mattress (that is information I linked to in my previous reply to you, but I know you haven’t had a chance yet to rad through all of that).
If you find out the specifications mentioned earlier, feel free to post back here and I’ll be happy to comment upon them.
Phoenix