I always preferred Mustang, until I got Ultimates.
I like the raised nose of their seats to keep me in the cockpit (the lowrider is more raised than the bigboy). (I have had a couple twinges of the groin over the years on the raised noses, but supportive underbritches and jeans removes this problem for me... the twinges were on really hot days in loose cotton pants).
I have both the bigboy and lowboy. I am 5'9" with short legs on 13" shocks, and I still prefer the allegedly 2" taller (and softer and wider) bigboy for comfort on long rides, but I prefer the lowboy for performance riding in the twisties for locked in the cockpit/seat reasons. The firmness of the lowboy and mustang are about the same (new or nearly new, old is always an unknown). I get my legs down fine on either seat, obviously the bigboy is a bit more of a reach, but no big deal.
The backrest does fold. I prefer the firmer and curved fit mustang driver's backrest to the ultimate backrest, and the mustang driver's backrest mounts right up on newer style flat-stock ultimate seat backrest slot mount. Mustang backrests also fold.
$500 for a four piece and a sissy bar, not just the solo.
The four piece (minus the sissy) is $845 new.
http://ultimateseats.ca/honda-motorcycle-accessories/valkyrie-standard-tourer-seats/4C-1001
