Treated Pine vs. Cedar which is better?
This answer is based on 35 plus years of building wood fencing in Tennessee. I have no reason other than experience to base my opinion. In Middle Tennessee we seldom build cedar wood fencing. Here is the reason why.
The simple answer is treated pine over the long haul is a better product than cedar {Western Red Cedar}. The only benefit to cedar is it {cedar} doesn't warp as bad as treated pine. It is not the pickets that warp so much as it is the 2x4s. Most fence companies will sell a cedar fence using treated pine framework {2x4 stringers and 4x4 posts}. Using cedar pickets doesn't really make a lot of difference in warping {if a treated pine fence is built properly}. It is the 2x4s that make the difference. I don't know of a fence that has been built using cedar 2x4s and treated posts and pickets. Over a long period of time that would be the best combination if you want to avoid warping.
The big drawback to cedar is the cost and the fact it will not last as long as treated pine. The pickets can cost twice as much and cedar 2x4s can cost 2 1/2 as much as treated pine. Cedar posts haven't been used in 20 years because they will rot in the ground as soon as a year after installation. The only time I ever use cedar pickets is when an HOA requires it {which is seldom}. Cedar pickets over time will rot, treated pickets will not. Cedar 2x4s will not last as long as treated pine. These are facts gather from experience. For the cost treated pine is the better product. Other than the warping treated pine lasts longer, costs less and if maintained will look as good as cedar.
The Warping Factor
For 20 plus year treated pine contained Chromated Copper Arsenate which in 2003 was determined to be hazardous to human health by the EPA. Since 2004 the preservatives used in pressure treat pine wood has been alkaline copper quat and copper azole. Outside the safety issue the good news is this new treatment does seem to have a better warping factor than the old Chromated Copper Arsenate. And when the wood is stained it does seem to hold as well as cedar when it comes to warping. Of course applying a stain or protective covering does cost but the benefits are better than cedar and the cost still remains less than cedar. My advice is to use treated pine materials for the best product at the best price. Add a protective stain covering and you will have as good wood fence as you can get over the long term
If you do want to build a cedar fence then it is important to use cedar 2x4s as well as cedar pickets. Of course you will have to use treated pine posts since cedar posts will rot {usually at the ground line}. Cedar often develops a fungi over time and the pickets will not last as long as treated pine. I suggest you use a protective covering stain to get the best results from a cedar fence.
Nails
Another consideration when building a combination cedar / treated pine fence is the nails. Galvanized nails will leave streaks in the cedar pickets, so you must use stainless steel nails which are twice as expensive. Just another reason to go with a pure pressure treated pine fence.