To furthur elaborate on Redsy's answer, If you look at Art 347 ('99NEC)(Rigid Nonmetallic Conduit) it starts off (347-1) with a description and characteristics of the conduit and then follows in 347-2 and 347-3 with the uses permitted and not permitted. This same information is in the NEC for each type of conduit, raceway, etc. (348 is for Emt)
As far as what best to use, that would depend on the situation. I'll bet in Arizona you can get some pretty nasty looking pvc sags! Joe T. had a great picture of that somewhere.