Hi Chris,
 
Medical advice for people suffering with back issues used to be that they ought to go for a firm mattress. More recently that advice has been contradicted and in fact now the advice is that you need support from the spring unit and the comfort should be provided by the upholstery layers. One must not get confused with tension and feel as they are separate entities. You can have a medium spring tension with soft upholstery layers for example.
 
Without knowledge of your wife's body weight, overall preferred feel and budget, it is not possible for me to make a specific recommendation. Please call our office with these details at which point we will be able to give more detailed advice. Kind regards.