It gets hard now to describe the feeling, as everyone has different levels for comfort appreciation.
On smooth roads there is definitely no problem with -20mm. On bumpy roads ( no pot holes) one can feel the stiffness... and it does feel like a stiff car. I like that feeling, so I enjoy my -20mm. I find the sport seats to be good enough to absorb the bumps and the hardness, but there again, it will depend a lot on each ones level of comfort need.
If you want to be sure not to be uncomfortable, take PASM.
If you are sure that you like a firm, very sporty ride and you don't mind to be sometimes shaken like a leaf, take -20mm. To me -20mm is not reasonable, but I like the feel much more then PASM on sport and when I drive fast , it feels so good...