Hi mbayer,
I’m not Brownie19 of course and hopefully they will see your post and reply but I would also make sure you can confirm the quality of the materials in the iSeries Expression (which may not be possible) because as you can see from the description here there are several inches of unknown and lower quality materials in the comfort layers above the innerspring which would likely be a weak link in the mattress in terms of durability. This can lead to the more rapid loss of comfort and support and the need to replace the mattress more quickly than you would have hoped and the loss of comfort and support isn’t covered by a warranty.
Just in case you haven’t read it yet … I would start with the tutorial post here which has all the basic information, steps, and guidelines you will need to make the best possible choices … and know how and why to avoid the worst ones (see this article). As you can see … I would avoid the iSeries Expression or any mattress where there are 2" or more of unknown or lower quality materials that will likely soften and break down more quickly than higher quality materials.
Phoenix