Hi Jeney,
Today seems to be the day for comparisons between SleepEz and Savvy Rest and I think that post #4 here should be helpful and apply to you just as much as to the previous poster that asked a similar question.
If you have tested a specific Savvy Rest layer combination in the store and it worked best for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then I would choose something that was as close as possible in all respects because changing the layer thickness of the top layer will change the feel and response of the mattress and every small difference in the designs of two mattresses you are comparing would make a difference in how they compared in terms of “feel” and performance.
With thinner top layers the firmness of the layer below it would “come through” more and have a bigger effect on what you feel on the mattress. In most cases the sleeping surface would feel firmer in a mattress that had a thinner top layer but was the same in all other respects and this would in turn have an effect on the comfort and support qualities of the mattress (neither would be better or worse … just different … and more or less suitable for different people that have different body types and sleeping positions). A softer layer underneath the medium would “act” softer than M/M in the top two layers but the thinner layer in the middle would also offset the softness slightly (thinner layers “act” firmer). The only way to know with any certainty how any combination of layers will feel and perform for you would be your own personal testing or sleeping experience which is the reason that the ability to re-arrange or exchange layers can be an important part of the “value” of an online purchase.
Phoenix