Hi Jenney,
All of these use good quality materials and have adjustable layers but they also have a wide range of pricing for similar materials. SleepEz is one of the members here which means that I believe they are among the best quality and value available but they are sold online so you can’t test them first (although you can exchange layers).
The choice between them would really depend on the parts of your personal value equation that were most important to you and the importance of testing a mattress in person. Savvy Rest and SleepEz both have organic Dunlop and 100% natural Talalay (not organic) available. Sleep also sells 100% natural Dunlop along with blended Talalay and offers layer exchanges if your choice of layers is less than ideal for you. The Bella Serra site says that their latex is USDA certified but this would just be the raw latex and not the core itself (USDA doesn’t certify the final product, only the agricultural raw materials) so you would need to ask them if it was GOLS certified as well. There may also be a difference in the covers which can be a significant part of the feel, performance, and cost/value of the mattress.
the better options and possibilities I’m aware of in the St Louis area are listed in post #6 here and this thread has some feedback from one of the forum members here as well.
Phoenix