If looking for smoother... I would stick with the Monroe super duty/extreme duty. I Installed KYB and combined with upgraded sway bars, bushings etc it combines to be a tad firm.
Tires become extremely important. With the firm suspension, The Nitto Motivo's help soften the ride. This is from experience as I have: 3 firm suspension panthers, one w Nitto Motivo's, one with stock KDS, and one with Nitto Gtr (or whatever they are). Not really fond of the newer Nitto. They are loud, (sloppy), abrupt, and drifts... Whereas the Motivo's are steady, quiet, linear, and reassuring in wet conditions. Perhaps a tad soft sided... But w firm everything else-a great ride.