When choosing a mattress, one must consider the opinions of one's cats.
Face it, you're only going to use this mattress for a few hours every night, maybe for the occasional mid-day nap. Who's really going to be using this mattress? Your cats, who are home alone all day resting in perfect comfort! So don't ask what mattress you need. Ask what mattress THEY need.
IF YOUR CATS ARE THE SNUGGLY TYPE you should consider a Twin Responda! It's got all the support you want, but it'll force your cats closer together because hey, it's only a twin! And that means just enough room for snugglin'.
IF YOUR CATS ARE THE LOUNGING TYPE you should go after a Full Responda. A full mattress will support all cats as they stretch their paws as far as they can go, while still offering a full range of motion as the sunbeam shifts from wall to wall. The perfect afternoon! Assuming you leave the blinds open a crack.
IF YOUR CATS ARE THE REGAL TYPE you should consider a Queen Responda. It's just large enough for your cats to sit up, survey the room, and pass judgment upon all. Then it's a quick nap and a long night of knocking the jingley bell back and forth until 3 AM.
IF YOUR CATS ARE THE POUNCING TYPE you should go after the King Responda. Then you'll be able to hide in the far corner of the bed while your cats stare at you with twitchy tails, waiting for your feet to move even a millimeter. Good lucky!
IF YOUR CATS DO NOT GET ALONG WITH EACH OTHER you want a California King Responda. Then they'll be able to ignore each other on opposite sides of the bed. All will be well. Until they meet at the supper dish.