How the load is lifted on
If the cargo is lifted from above by a crane or gantry — steel sections, crated plant, pipes — a flatdeck is the only sensible option because a tautliner's roof is in the way. If it is forklifted from the side or rear, a tautliner works well.
Weather and security
A tautliner protects against rain and road spray and hides the cargo from view. A flatdeck offers neither unless the load is wrapped or sheeted. For packaged consumer goods that usually decides it.
Shape tolerance
Flatdecks accept awkward shapes, overhang and heights that will not fit inside a curtain body. Tautliners are constrained by the roof height and the internal frame.
Cost
Rates are broadly comparable for the same configuration and distance. The real cost difference comes from loading time and whether you need sheeting, additional securing or permits.