Drop in bedliners from reputable manufacturers (e.g. Carryboy) are usually made of plastic.

Drop in has the advantage of preserving your original paint. However, water can get trapped between the drop-in liner and the pick-up bed. This can lead to rust. Not always, but it can.

Spray-on bedliners are applied by first scuffing (ginagasgas) the original paint of your bed. Then they spray it on. 'Nagbabakbak' depends on how good the prep was done and what kinds of stuff you carry on your beds. Spray on liners can sometimes handle corrosive liquids (e.g. acids) even better than drop-ins.