I don't know about other places, but around where i live, PVC is impossible to get in different sizes. ABS however is all over the place. I also know that it is very flexible, and EXTREMELY pressure resistant, whereas PVC will shatter. The PVC sliders i've seen all have wood reinforcements inside, i would imagine ABS wouldn't need any since its so much thicker. and it's also very flexible. I vote ABS all the way. (althought its black and not as visible).
as far as the "slider" vs "grind-rail" debate.. would you rather SLIDE your board or grind it into oblivion?? nuf said.