With the company I work for, we manage a condo complex with cedar trim. We are replacing boards all the time. My guess is by the next five years all the cedar board will be replaced with Azek. But, the way the builder installed the cedar boards is keeping money in my pockets. The builder did not seal the end grain on the cuts. Nor did they use any caulk. The only side sealed is the face of the board. And they didn't prime the boards!! So we are replacing a lot of rotting boards. Oh and no tabs on the J channel...but that's a different topic!
So try to seal the end grain of your boards and use a good primer, and I don't think you could go wrong with fir or cedar. As long as you maintain it properly.
Just my two cents.
Kapex with UG set, DF500 , HL 850, OF1400, MFK700, TS 55, PS 300, CXS, C15, MFT 800, MFT1080, (3) MFT/3, CT 26, CT midi, LS130, DX93 , DTS 400, RO 125, ETS 125, LR32 set, VS600 dovetail jig, parallel guide set, FS 800, FS 1080, (2) FS 1400, FS 1900, FS 3000, FS 1400-LR32, FS 2424-LR32, Syslite, centrotec installers set,