Hi PattyS,
I would probably suggest a “reset” and start your research with post #1 here which has all the basic information, steps, and guidelines that can help you make the best possible decisions.
If you follow the steps one by one you will likely end up making a very good choice.
A mattress is only as good as its construction and the quality of the materials inside it and unless you know the specifics of all the layers in a mattress you are considering there is no way to make an informed decision or make meaningful comparisons between them. Both of the options you mentioned are questionable in terms of quality and value.
The Serafina uses 3" of 3 lb memory foam which as you can see in the memory foam guidelines here is lower quality/density than I would consider as a reasonable option unless the durability of your mattress is not an important part of your purchase.
The Spring Air Natalie Hybrid doesn’t disclose the quality of any of the materials at all and I would avoid any mattress where the quality information isn’t disclosed and there is too much lower quality polyfoam in the upper layers of the mattress which can soften and break down much too quickly.
I would be particularly cautious about using reviews as a way to make a mattress choice and you can see more of my thoughts about reviews in post #13 here.
You can also read more of my thoughts about buying from Costco or any Big Box store in post #4 here.
IMO … there are many much higher quality and value choices available to you than either of the options you are considering both of which I would tend to avoid.
Phoenix