One distinct pleasures of this trade is the use of many interesting materials. On the ones below I'm using MDO and Rough Cedar. Both are long lasting when prepared properly.
MDO is a strong substrate that doesn't warp and will last for years but the edges are always the weak point. When sealed up properly and given three coats of paint they will last a good ten years plus. Good old Rough Cedar is one of my friends. I'd rather work with it over Redwood.
For letters, especially white one's, I like PVC.
Joe