Hi edh,
It seems to me you are down to choosing between “good and good” choices even though they are somewhat apples to oranges.
The Serenity uses 5 and 8 lb density memory foam and a 2.2 lb base layer which are all high quality materials. It is a firmer mattress and of course it is also more costly because the quality density of the foams are higher. You have tested this mattress so you have a good idea of what it feels like. the quality of the materials are not normally found in this price range.
The 13" gel foam uses 4 and 5 lb memory foam and has a 1.5 lb base layer. It is softer and of course much less costly (about half the price) but the density/quality of the foams are lower. It is just slightly firmer than the Tempurpedic Cloud Luxe so if you test the Cloud Luxe you will have a sense of the difference between the “feel” of each mattress which of course would be an important part of your buying decision. Of course this is also great value and again you don’t normally see this quality of memory foam in a mattress in this price range.
You are down to good choices and your thoughts about how they each feel and perform and the objective, subjective, and intangible parts of your personal value equation that are most important to you are the best way to decide between them.
In quality and value terms it would be difficult to make a mistake even though they have a very different feel which will probably play a significant role in your choice.
All you have to do is email them, call them, or use their chat on their site and let them know your membername here before you make the purchase and they will give you the code.
Phoenix