you can adjust a normal traxxas gear diff with different oils, no?
a ball diff is definatly an upgrade for a racer as you can tune it
without pulling apart anything (pop a driveshaft out thats all).

not worth $50 though in my opinion and not for bashing obviously
the gear diff is tougher.

just a note most ball diffs set up correctly and maintained well, are
more than capable of taking brushless power, and more to the point more
power than is allowed in most classes of racing.