Which you get depends on what you play, if you mainly play caster classes (HW/HP/Prof/etc) or a ranged class (Archer/slinger/etc) stings are more useful.
If you play a lot of other jobs as well then berzes are more useful since they provide more reduction for all classes.
Personally I'd get berzes. They're useful for practically every classes while Sting gloves are much more limited in terms of usability. Cause that +10 DEX won't really help you if you play a Pally or a SinX, but 60% cast reduction is a lot more useful.