Think I posted this pic before somewhere, but what the heck. Here's a good comparison between the stock 1199 and 899 seats. The 899 seat is a bit taller and grippier, but it's softer so it doesn't raise you up too much over the stock seat when your weight is on it. Planning to shave mine down and reshape it a little bit, but it's a better base to work from for that.
I tend to put the stock seat on if I'm going out scratching for an hour or two since it's slimmer and easier to move around on, but run the 899 seat if I'm going longer since it's definitely kinder to the rear end.