It ain't what you got...it's what you do with it.

I agree with John about choosing the timber carefully, the same is true for fir: go for high summerwood content and nice, even growth, avoid the sapwood and pith. Both WRC and D-fir can be used effectively outside and there's plenty of historic precedents to justify this...but both can deteriorate pretty quickly in certain situations too. Even the sweetest cedar frame will eventually rot away if water can dribble down the knee braces and stay trapped inside joints, or if the post bottoms can wick moisture up from unsealed concrete. Longevity is dependant upon: good material selection, good detailing (especially around post bases and eaves) and a little basic maintenance. Specifying the right species isn't enough on it's own...and every exterior frame needs to be looked after a bit. Whichever species you decide to go with, ensure that the timbers can always shed water, 'breath' (good air circulation all around), and dry out after a soaking. Likewise, make sure that the proud owners know to keep your hard work free of leaf/tree debris or anything else that might trap moisture.