Foam mattress choice

I'm looking to change my king size mattress and am after a super soft one that I sink into. One that doesn't bouncy me around when my husband gets in or moves about. I'm 5ft9 and 70kg and my husband is 6ft3 and a little over 100kg. We were looking at the Mammoth sky 270 but as you say until we've tried it properly we can't be sure it's right and I don't want to spend a rediculous amount of money for the bed to be wrong! Any suggestions? I was looking at your Hybrid 4 but I'm not sure that's the right one??