Well its like this. The pillion is thin, a little narrow and kinda firm which contrasts with her backside. The Sundowner is wider, and softer which is more of a match with her backside. Don't tell her I said this.
Also, with the detachable backrest on, the sideplates dig into her rear. Since the Sundowner is thicker, it raises her up a little and the side plates don't bother.
Mustang....The vintage solo with the 13.5 inch wide pillion ( i Think thats the widest one) is the way to go. My wife and two girls 25 and 18 all agree. The width and thickness makes all the difference in the world and the base of the seat is made of steel....thats a a big deal on a long ride. The harley stock seat bases are made of thin plastic. Junk. Check out the underside before you buy................ Also...plush or poofy means your butts gonna hurt soon........Hard or dense padding is more comfy on a long ride. My 2 cents....
I bought a Vintage Mustang solo 2 weeks ago for my '07 Fatboy and my butt couldn't be happier. Rode just over 160 miles non-stop and NO aches ans pain. Good buy and you might consider getting the optional back rest for the really long rides.
When riding two up I use my stock seat and pillion.