Hi aoysgelt,
Part of the difficulty with just using data is that it never applies to an individual, only to averages and groups, and just like any group the individuals in it can be very different and even the differences can be different for many reasons. Different needs and preferences can be based on individual sensitivities to certain elements of mattress construction or on physiological differences that may not be obvious. For example one person with more or less flexibility in certain joints or with a certain spinal curvature or body structure may do very well on a particular mattress while someone else with a very similar body type and sleeping style may not do as well. In addition to this, individual subjective preferences can also play a big role in how well someone sleeps and there is no way to quantify many of these. Hidden differences can also play a big role and one person may need the freedom to move more easily because they are not as comfortable in one position for as long while others may tend to stay in one position for much longer. This can be a result of obscure factors such as different amounts of soft tissue covering some bony prominences or different intra capillary pressure in different people or body areas.
All of this is to say that only the experience of a particular person can tell for sure if a mattress is “right” for them and everything else is just increasing or decreasing the odds that your experience will be what you want and need.
So to relate this to your circumstances … you will probably have a better idea of the type of mattress that your wife prefers than most people and you would also know how her preferences feel to you. For example I could probably go into a store and lie on a mattress and “guess” whether my DH would like it or not. I may be wrong but the odds that I would be reasonably close are fairly good.
So there would basically be two advantages to buying locally. The first is that at least one of you could be more accurate with your specific needs and preferences. There are many differences between different memory foams or different layer combinations for example and these differences can be difficult to describe without actually feeling or experiencing them and they may actually perform or feel very different to different people (some may call a certain mattress too soft while for others the same words can have a different meaning or they may be focused on different properties of the mattress and call the same mattress too hard). Someone local has the advantage of seeing how you respond to specific mattresses in “real time” which can help in narrowing down the types of mattress that work best for you.
The second is that there may be some local outlets that have the same or better value as an online purchase or their after sale options and costs may be better (things such as comfort exchanges or the ability to make adjustments to a mattress for a reasonable cost after you have bought it). The cost of shipping an online purchase back to do a comfort exchange can be very high while having it delivered or picked up locally can be much less. Of course each outlet is different and for example some online outlets can do layer exchanges instead of having to ship the whole mattress back again if you need to make an adjustment. For people who can sleep on anything an exchange or adjustment after a purchase may not be a significant part of value at all.
In general … an online purchase can be riskier than a local purchase (depending on the policies, fees, options, and services of each outlet) and each of your online vs local choices may have advantages and disadvantages that each person would need to take into account in their “value equation”.
I personally would tend to make a few local phone calls or visits as a matter of course because if nothing else local testing can help an online purchase be more accurate (for at least one of the two of you and because of your knowledge of her preferences possibly both). You will have experience on certain known layerings (for example you may find that a mattress with 2" of 4 lb memory foam over 2" of 5 lb firmer memory foam over 6" of firmer base foam worked well for you) that can help an online outlet help you more effectively. Even testing a widely known mattress like specific models of Tempurpedic which have known layering may help you increase the odds that an online outlet can help you choose something that is “somewhat” similar. Local testing can improve the odds with an online purchase and online outlets and the reference points they provide can also improve the odds of finding local value.
So in the end … “value” has many components from the materials, quality, construction, and price of the mattress itself to the knowledge and skill of the person helping you make your best possible choice to the services and options and cost involved if you need to make any changes after the purchase and even the type of business you are supporting with your purchase. All of these contribute to the advantages and disadvantages of dealing with different types of outlets and can be more or less important parts of each person’s “value equation”.
The goal is always to do as much as you can or that is practical or worthwhile for each person to increase the odds that what you receive has the components of “value” that are important to you. There’s no right or wrong in any of this … just changing odds of success and long term satisfaction.
One of my “jobs” and goals is to help people think about and identify what may be more or less important to them and help them make more informed assessments so that the parts of their “value equation” that they may not otherwise realize or even think about till after their purchase can be part of what they are looking for from the start 
Phoenix