Hi aminah,
I think that one of the most difficult things for most consumers to understand is the amount of technical knowledge, intuitive knowledge and experience can go into the choice of the “best” mattress for any particular person.
When you are dealing with a good online retailer or manufacturer (or local as well) … their goal is always to help you make the choice that they believe will make you the happiest in both the short term and the long term. The goal of both the merchant and the consumer is to get it right the first time so that there are no exchanges involved or worse yet (for both) a refund for those that offer this. This can take considerable expertise. In many cases when you talk with them they will pick up on some cues that you may not even realize you are sending that can make a difference in what they suggest. They are also well aware of both short term and long term needs and preferences that a consumer may not be as aware of. For example someone may test a mattress locally and then provide the information about how it felt and how they say it or in some cases what they don’t say can be as important as what they are actually describing. In other cases they may believe that what they are suggesting will be mostly similar but will have some differences that may be difficult to describe but will have much better odds of making you happy in a few months and years from now as it does at the beginning (and hopefully even more so).
What this means is that you could talk to 3 different manufacturers that could each have a different suggestion and all of them will genuinely believe that you will do better on the suggestions they are making even though they may be different. This is why part of the value of an online mattress purchase is the recourse that you have in the event that you make a less than ideal choice. This is also why the ability to both rearrange or exchange layers or the return or exchange options of the mattress with an online purchase can be an important part of “value” in case “best efforts” are not quite close enough. They are also much more familiar with the smaller details of the layers and components that they offer that can make a bigger difference than their customers may suspect and they will each have dealt with a large number of customers that would be similar to you at least in general ways and made certain choices that needed to be changed after a purchase. Each merchant may pick up on different things in a conversation or sometimes smaller differences in designs or materials may lead to different suggestions as well. Part of the “value” of any mattress purchase is the value, knowledge, and expertise of the person you are working with.
As hard as it may be for many people that haven’t spent years in the industry selling mattresses to really understand, there are many different possible combinations that may work very well for each person that may each have different “tradeoffs” and there may not be a single “best” combination.
So my suggestion is always to talk with each retailer or manufacturer you are working with and ask them any questions that will help you resolve any uncertainties you may have about the “why” behind what they are suggesting. In the end of course you could always “duplicate” what you tested as closely as the materials they offer make possible (if you are absolutely certain that what you tested is your “ideal” in both the short and long term) but this may not always be in your best interests.
In this case as well … each mattress uses a completely method of adapting to two different body types and this can also affect the layer choices that may be best and can take some experience to translate one into the other. Changing the order of layers, the amount of layers, the thickness of the layers, or the softness of the layers can all affect how a mattress feels and performs in sometimes unexpected ways because every layer and component has some effect on every other layer and component in a mattress.
I can certainly provide some of the 'theory" behind different types of layering but in the end it’s always best to work with each merchant you are working with to come to a decision about which layering they offer is the most likely to be “best” for you. They are the most knowledgeable “experts” in their own designs. The objective/technical part, the subjective/intuitive part and even the intangibles that are just based on “gut feel” are all part of this.
The thing I would always remember is that everyone’s goal is for you to end up with a mattress that is the “best” mattress for you and of course I would also factor in your own confidence in your purchase as well as the value of being able to re-arrange and even exchange layers if that is necessary.
To answer your question about Talalay vs Dunlop … if you compress each layer more than 25% then Talalay will typically feel softer in the same ILD as Dunlop yes. Sometimes this may not be a completely accurate comparison because ILD itself is not always as accurate as many people believe and other qualities of each type of latex play a role in how each person perceives “softness” or “firmness” as well. There is “hand feel” softness/firmness, “pressure relief” softness/firmness, and “support” softness/firmness and all of these may be different (see post #15 here). Sometimes people talk about one when what they are describing may be more about another. There is more about the differences between Dunlop and Talalay in post #7 here.
So the “best” advice I can give you is to do what you do best which is some local testing which is as accurate as possible which you can describe as specifically as possible along with asking all the questions that are necessary for your confidence level to be high enough to “pull the trigger” on a purchase. At the same time I would let them do what they are best at which is to listen carefully and to make the suggestions that take everything you have discussed into account (along with the things that you many not even know to ask), answer all your questions, and use their knowledge and experience so that your final choice has the best odds of being everything you want it to be. This is the shared goal of both of you.
If you are dealing with one manufacturer … then this is simple enough because your best choice with that manufacturer becomes your purchase. If you are dealing with more than one manufacturer … then once you have made your final choice at each one … then you are down to choices between “good and good” and there are no clear winners between them then your final choice (see post #2 here) will be a matter of using all the information and knowledge you have gathered, along with your “educated intuition”, and “gut feel” to make a best judgement decision based on all the parts of your personal value equation that are most important to you (including price and exchange or return options of course).
This is why both the design and manufacturing of a mattress and the choice of which one is most suitable for each person is equally an art and a science … which is also why it’s always so interesting 
Hope this helps.
Phoenix