Cedar, usually white or western red, is a dense, yet lightweight material with a natural tendency to resist rotting and infestation as long as it is kept above ground contact. While cedar is a more stable material (i.e. amount of warping & twisting) it does not have the longevity of pressure treated.
Pressure-treated lumber,usually southern yellow pine, is a semi-dense, heavyweight material with a combination of chemicals injected into the cells of the lumber during a vacuum process at the mill. This chemical, Micronized Copper Quaternary or MCQ, is environmentally safe and delays any rotting or infestation from occurring during the life of the lumber. This provides for outstanding durability and longevity in fencing and decking materials.