Get them both. They make a big difference in the way the trailer tows. Smoother with less chucking. Had the Trailair suspension installed a on my Sabre, made a big improvement. Got both the suspension and the Roto Flex box on my Columbus, really big improvement.
2013 Palomino Columbus 320RS
2007 Dodge RAM 2500 6.7 Cummins
Reese 15K manual slider
M.I.L. in Florida for good!!