Hi ryk,
Post #3 here talks more about the 3 ways that it may be possible to “match” one mattress to another and I don’t think that any of them would be really possible here. I would also keep in mind that firmness and softness itself can have different meanings to different people (see post #15 here about the different types of softness) .About the closest you could get would be for you to estimate how firm or soft the Serta iSeries pillowtop felt for you on your own personal “firmness scale” (from a hard floor to the softest you could imagine) and then talk with Chuck about which choice on their firmness scale may be similar. This would be a very subjective exercise and would depend on how similar your two scales were and on whether you were talking about the same type of softness but it would be as close as you could get. I would also bear in mind that the firmness/softness that you tested may not be the one that is best for your body type and sleeping position unless you tested specifically for PPP (Pressure relief, Posture and alignment, and Personal preferences) when you were in the store (see the links in post #1 here about testing mattresses for pressure relief and alignment). Besides just the overall “feel” of a mattress it also needs to “match” your body type for both comfort/pressure relief and support/alignment and for some peoople the Serta iSeries pillowtop could be a very risky choice in terms of alignment because it is the softest of the line.
Post #2 here talks about the main differences between them. For most people (but not all) they would “feel” very similar with the same comfort choice. In case you haven’t seen them they also have the new line they are launching on their site and the Camilla there also has a replaceable layer at a lower cost than the Eurotop on Amazon.
Some of the better options I’m aware of in the NYC area are listed in post #2 here and there is a more “categorized” list with more detailed descriptions for some of them in post #7 here.
Post #4 here also has a list or “possibilities” centered around Setauket in case any of these are close enough to you to be worth visiting.
Phoenix