Sight unseen, from the 8", to 10" to 12" thick memory foam mattress, the thicker it is, the softer it will sleep. Checking out NovaForm's website, you may want to be sure it's a product made in the USA. The 20 year limited warranty does not cover much and if it's produced in China, you shouldn't have the anticipation of it lasting as long as your last mattress did. There are better quality beds made in America you can purchase and at the cost of a higher price, there is a better guarantee you will get at very long lasting mattress. At your request, I can link you a few.

The thicker, softer models are more geared for comfortable side sleeping while the 8" would be better for stomach and back sleeping. The 8" may bottom out a little bit for side sleeping which wouldn't allow for long periods of restful sleeping on your side.

When buying a mattress this way (and I don't really advocate it) you're left with just the statistics .I agree with what Andrew said above. Look at the density (lbs.) of the visco foam and it's thickness (not just the overall mattress thickness but just the visco). In general thicker and heavier is better for durability. Also look at the fine print in the warranty for how much sag or body impression is allowed. 1.5 to 2 inches is industry average (Tempur allows only .75 inch! Really great). But as Andrew mentioned if the manufacturer is in China you may have no recourse in the future even at Costco. Just a thought but would you buy a car without being able to test drive it? There are less than Tempurpedic price visco/memory foam mattresses in many mattress shops. Take some time and try a few of those while you're at it. You're going to have this bed for a few years so you better like more than just the price. Happy Shopping and pleasant dreams!

As much as I love Costco and Sam's, my advice would be to NEVER purchase a bed from them. Here's why, First, in order to judge a bed accurately you should lay on it for at least 15 minutes. I have never seen a display available to do that. Second, have a question, and you do, who is there to answer it? No one. Third, you will be tempted to tie it to the top of the mini van, don't do it! If you have a large enough vehicle to place it in, preferably covered, ok, otherwise have them deliver it. Third, most mattress retailers can deliver AND pick up your old bed, and pricing is not that much different. The added plus is mattress store sales people know more about their beds than Costco's produce or stationary managers, and you would know the answer to your question by now. Fourth, I always recommend buying as local as possible, local merchants have more of a stake in your community, and they are more likely to better care for you and your purchase.

As far as the mattresses you selected, it is impossible for any one to tell you which you would like, we are individuals. Until you lay on then you won't know. Is there a return policy? if so try the one that is medium thickness, if it is too soft return for thinner mattress, if it is too hard, return for thicker. Which kind of returns us to the first paragraph... I would urge you to buy a bed for the price AND the feel, afterall you had the last mattress for 20 years, and you spend a third of your life in bed. And I have found out that mattresses, like most things in life, you get what you pay for. The competitive nature of the industry will make low quality mattresses the least expensive... and the reverse.