Redwood
Redwood is a colorful, attractive, durable wood, generally resistant to rot and insect pests. It is used for decks, fences and similar purposes but is not classed as structural lumber. It is used most often in fences for posts, because of its resistance to moisture and insects, and for fence planks in solid board privacy-type fences.
Douglas Fir
Douglas fir is primarily considered a framing lumber, because it is stable and structurally very strong. The highest quality Douglas fir comes from the Pacific Northwest, west of the Cascade Mountains, although it also is grown in Colorado, Arizona, New Mexico, Utah and Nevada. In fences it most commonly is used in post and rail styles, rather than solid fencing.
Preservatives
Both redwood and Douglas fir are sturdy lumber, which hold nails and screws well and are strong and resilient enough to resist strong winds. Douglas fir fence posts are usually treated with a preservative against rot and insects because the wood lacks the natural resins that shield redwood from these problems.
Options
In examining redwood and Douglas fir for fence applications, the style of fence, the location (such as potential for moisture) and availability of the lumber will be factors. Redwood is favored for a solid fence, Douglas fir for post and rail or "horse" style. Redwood is best for any location subject to frequent moisture. Douglas fir is more available in most areas; redwood is most available and cheapest in California and neighboring states.