Ideal mattress and variable weights

Hi - great website, spent most of the morning reading up on mattresses and the afternoon at a retail park trying to get a feel for what difference it all makes. My partner is 6ft and 15 stone, I'm 5ft 2 and about 8 and a half stone. Having tried a few out, I'm leaning towards a medium - soft pocket sprung, the other half is leaning toward medium. He's had a bad back for years, and I've recently recovered from a shoulder injury. We think we've narrowed the search down to either your super King Artisan Naturals or or Artisan Luxury. We can't decide on whether we need different tensions in one mattress or a split and zip ? If we do opt for a zip and split would we need a topper to cover the split ?