similar weight but top end of medium and lower end of firm, what best to suit?

Currently reviewing Artisan Naturals range, double bed, pocket sprung base. one spouse is 97.4kg top end of medium spring tension and the other spouse is 110kg lower end of firm tension. Inclined to purchase the firm spring mattress, as hoping that the choice of pocket sprung base will create a softer feel. We simply don't have the room size for a king zipnlink. I realise that the double will be slightly firmer than a king size per springs count. Do you think opting for the firm is the right way to go or, would the medium be too soft for the 110kg person, as technically this would be beyond the tension etc. We both favour a firmer bed though lower back (110kg & 97.4kg) and shoulder pain (97.4kg) encountered. Side, back and occasional front sleepers! probably down to discomfort every day... current mattress on a platform base and is a 1000 pocket sprung, though double so less springs etc, said to be orthopaedic mattress. Your opinion would be gratefully received.