Hi newalf,
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices … and learn how and why to avoid the worst ones (and both of the mattresses you are looking at would be on the “avoid” list based on the guidelines here and the list of the top 15 manufacturers here).
Outside of careful and objective testing to make sure a mattress is a good “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences), a mattress is only as good as it’s construction and the quality of the materials inside it (regardless of the name on the label) and the most important part of a mattress purchase is making sure you know the type and quality of all the materials and components inside it (see this article). Without this there is no way to know whether a mattress has any weak links or make meaningful comparisons with other mattresses.
While it’s not likely that you will be able to find out the information you need to make an informed choice with either mattress (neither Sealy or Serta or the retailers that focus on them tend to be very open or transparent about the quality of the materials in their mattresses) … if you can find out the specs of both mattresses and list them on the forum I’d be happy to make some comments about the quality of the materials or the mattresses in general.
Without this information it isn’t possible to make an informed mattress choice or for me to make any meaningful comments about the quality of either mattress and you would be making a completely blind purchase.
I would also make sure you read post #174 here about mattress warranties so that you are clear that they have little to do with the useful life of a mattress or your ability to replace it if it loses it’s comfort and/or support because of the premature softening of any lower quality materials in the mattress (which isn’t covered by a warranty).
Phoenix