When I bought my Fatboy, I knew that the seat would be replaced eventually. With the stock seat, I'm only good for about 50 miles before the back of my butt starts to feel un-comfortable. After 100 miles, I have to get off for a while and walk it off. A friend of mine says his stock Heritage seat is the same way for him. When I asked the accessory pro at the dealer what he recommended for a comfortable touring seat, he went right over and grabbed the "Sundowner". It's one piece with smooth lines and it felt a lot softer. I'm not sure who makes these for Harley but the price tag was around $265.

 

I have had the stock 2000 and up Heritage seat on three bikes. It was comfortable to me. I even took a long, 7000 mile ride, up to 700 miles a day with the stock seat. However, I just put a Sundowner seat on my '01 Heritage and love it. Gives a bit more back support and has a deeper bucket.
Best part is I bought it on Ebay for $110 including shipping.
I have tried the Mustang Touring seat but it sits me up to high and is to wide. I am short and had trouble planting my feet flat on the groung.