I can tell you that the hd seat is not very 'substantial', but I don't know how it compares to the mustang. also, the hd seat is vinyl.
I don't know where you're getting the hd seat from, but msrp is $359
for almost the same seat, consider the cvo solo seat. almost identical, except the seating surface is leather.
quick reference for part numbers:
52000042 black/grey seat
52400024 black/grey pillion
52000043 brown/gold seat
52400025 brown/gold pillion
52000056 black/gold seat
52400038 black/gold pillion
I've done several long trips on mine, and while my ass does get sore after a while, I don't think it gets any more sore than it would on any other seat. it isn't so bad that a quick walk at a gas stop is plenty to alleviate it. no issues with over 12 hrs in a day in the saddle.
for the best pricing, contact jon at hornytoad hd on these seats. I've purchased 2 of them over the past couple of years. service and price can't be beat.