PBT keycaps are usually slightly rougher than ABS, however they do not shine over time, whereas ABS could start going shiny in a couple of months, depending on usage.
There are a few PBT doubleshot sets appearing now, mostly by Vortex, but the majority of PBT are dyesub. Most ABS are doubleshot.
Like most keyboard-related things, this comes down to personal preference.
Some people swear by ABS, some by PBT, and some even by POM.