How much give do your mattresses have in them?

Hello, I'm 16 stone and 5' 9" and am looking to buy a mattress. I prefer a very firm mattress which doesn't really let me sink in at all. Any mattresses with foam toppers or which have thick padding on them which allow for much sinking in I find uncomfortable, but if mattresses are too unyielding then my wife (who is about 2/3rd of my weight and just under 5') finds that too hard. She finds a firm mattress comfortable, as long as there is a bit of padding on it. We had considered getting an ultra firm mattress to keep me comfortable and then adding a single topper for her to have her side of the bed which would give her the extra padding she'd want, but what would you advise?