What type of Piping for Reduced Pressure Backflow Preventor for Irrigation System
Copper can have problems with high mineral content but even with that it will last decades if prevented from freezing and I think it is the best for exposed work it is easy to cut and sweat fittings and my top preference.
With PVC if doing exposed work I use schedule 80, I have not had problems with sunlight compared to PVC in exposed areas getting broken especially in winter. 
Steel would be my last resort, it is harder to cut and thread but holds up quite well on exposed work. As far as rusting it will take many years, I have threading equipment but find both copper and PVC hold up quite well and the extra work of threaded pipe so not worth the time and extra work for me.