Hi shh64,
This is an ongoing debate that is really a matter of personal preference and there is no “better or worse” answer. You can see some of the pros and cons of each in this thread and post #2 here and post #16 here are also about the same topic or “debate”. More than anything though … it really depends though on all the different factors that are part of each person’s “personal value equation”.
The “value” of a mattress purchase includes many things. One of these is the “commodity value” of the materials in the mattress, one is the suitability of the mattress and the knowledge and experience of the manufacturer or retailer to guide you with this, one is the benefits, services and extras that come with the mattress, one is any options you have (or don’t have) if you need to make any changes or exchanges after your purchase, and then there are any other tangible or intangible benefits that go with each choice that may be important to each person.
There is no doubt in my mind that personal testing is the most accurate way to choose a mattress and I would always start locally both to gain personal experience with different materials and combinations and to find the best “local” value. I would then compare them to any online options that are available to you. I tend to use a “local premium” in the range of about 20% or so to determine “equivalent value” but each person has a different risk tolerance and can use their own percentage.
Basically it boils down to the specific benefits of each choice (whether online or local), the amount of risk involved and the risk tolerance of each person, and which of the many tradeoffs involved in any mattress purchase are most important to each person.
Phoenix