Mattress for slats vs mattress for a divan

We are befuddled after a morning looking at mattresses in Hedge End. We liked the firmness category 3 best (Bensons and Bradbeers) and we liked the Tempur original but very expensive as we need 2.
 
Should we be looking at 2 different kinds of mattress as we need 2 mattresses- one for a pine slatted bed and one for a divan?