I have had plastic liners in my previous two trucks before selecting Line-X for my current truck. Each has their pros and cons, and what type of cargo you intend to haul may also have a bearing.
If you tend to haul "delicate" items that you don't want scratched up or heavy items that may tend to roll and potentially dent your bed (think engine blocks and transmissions), then a plastic liner may be for you. A spray-on liner will have a rough texture that helps to keep most types of cargo from sliding around, but that same texture will also rough up (scratch) soft objects that are dragged on it (finished wood, vinyl, etc).
For my Line-X, I need to have moving blankets or flattened cardboard on hand when loading up "sensitive" cargo.