profile, thickness/weight, and texture generally matter more than material.
difference between the materials is: abs commonly comes in double shot, whereas pbt comes in dyesub. double shot allows the legend to be ANY combination of colors, including translucent. dyesub keys can only have darker legends; there's no such thing as white dyesub legends because white is darker than nothing.
there ARE doubleshot pbt keysets, but they are relatively rare.
as well, pbt spacebars are harder to find because the material bends as it cools and this is very obvious on such a long key.
also, pbt is harder to shine. there are people who say that isn't true, but that
s horse**** as far as i'm concerned.
this list is NOT exhaustive.