Not all DP's ride the same. First off, stay away from the Roadmaster R4R chassis. They have a very harsh ride, even if the after market ride enhancement kit is installed. I speak from experience.
Generally speaking Freightliner and Spartan are very similar in ride and overall will give you a nice ride.
I've never pulled a 5th wheel, so can't compare the rides.
Also keep in mind, the wheel base will have an effect on ride.
Tire pressure will also effect the ride.
My suggestion is test drive different chassis. Don't just take them around the block, but get them out on the freeway up to speed and around some truck traffic to see how the react. Also if possible drive them up some grades.
Also drive them over some rougher roads so you get an idea of ride.
Test drive, test drive, and test drive.