Burgerinn1.) chemical composition of the plastics. 2.) Personal preference.
ABS and PBT are both good for different reasons, it all comes down to personal pref.
From my understanding, PBT is typically more robust, has a higher melting temp (hence why you can dyesub PBT but don't see dyesub'd ABS keycaps) and also has a different texture (more rough, whereas ABS is smoother/shiney-ish? May differ from manufacturer to manufacturer) however I have also heard that GMK makes ABS keycaps that some people will prefer over PBT because it's simply a higher quality ABS and better manufacturing process/practice.
Please feel free to correct if I am off base.
Im picking up the set because A.) I dont have a GMK set yet - B.) This set looks amazing and - C.) Did I mention this set looks amazing?
KM1337ABS cannot be dye-sublimated because it lacks the crystalline structure PBT, and some other plastics have. It doesn't have much to do with melting point.