Why drop-ins trap rust
A drop-in is a loose plastic shell. Water, sand and salt work under it and sit against bare metal, which is exactly where beds rot out. You often do not see the damage until you pull the liner years later.
Why spray-in wins
Spray-in polyurea bonds to the metal with no gap for moisture. It will not shift, warp or trap grit, and the textured surface grips cargo. It also deadens noise and takes impacts a plastic shell cannot.
The cost difference
A drop-in is cheaper up front, but a spray-in liner protects resale value and never needs replacing. For a truck you plan to keep or sell, spray-in is the better long-term buy.