Hi Betsy,
Before you consider any mattress … especially these ones from Costco where the quality is low and you will be dealing with people who have very little knowledge about mattresses or how to make a choice that is suitable for your body type or sleeping style … I would read post #1 here and the information it links to.
One of the most important parts of any mattress purchase is to make sure you know the density of any polyfoam or memory foam in the mattress. None of the options you are linking to include this information and without this there is no way to know the quality or value of the mattress and of course without some expert guidance you could easily end up choosing a mattress that is completely unsuitable for your needs and preferences. For example you can read a bit more about the Sleep Science Emma here (your second link) and what you would have ended up with had you made a completely blind purchase without knowing the quality of the materials. Novaform also tends to use low quality materials bu either way I would want to know what they are before considering them.
I would also suggest reading this post from today from one of our members which includes their experience with an inexpensive Walmart memory foam mattress that was highly rated (and reviews say very little about the quality or value of a mattress).
Once you have read some of the basic information you will need to exclude most of your worst choices (including any mattress that doesn’t list the quality of the materials that are in it) then I would connect with some of the better options that may be available to you locally where you can actually test a mattress and find out if it is suitable for you in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences). Even if you do end up making a purchase online, local testing on mattresses is an important part of learning about the type of mattress and materials that would be suitable for you.
If you let me know your zip or the city you live in I’d be happy to let you know about any of the better options or possibilities I’m aware of in your area.
Phoenix