The Rest Assured mattresses are easy-care no-turn and only have a single sided sleeping surface. You should, however, still rotate these mattresses from top to bottom to ensure even settlement. Single sided mattresses are not a detrement to the expected lifetime of the mattress.
The Adlington is available with memory foam or latex for a luxurious feel. The Royal Ortho will have a firmer feel to either of these two. As you existing mattress is an Ortho this would also have been more firm that others in the range at the time.