Here's my thoughts and observations on the subject.
I'll use the TA04 as an example.
While the standard planetary gears do feel very smooth on the TA04 in reality I think they are not as smooth as the Ball diff option IF the Ball diff is adjusted properly.
I suppose if you look at the design of the two you can see that the standard planetary gear diff has cast metal gears which create a lot of friction due to all those teeth meshing and the fact that the 3 smaller bevel gears sit on a metal shaft and rotate without and form of bearing etc. There are a lot of surfaces coming together.
In the case of the Ball Diff you have a much less complicated design with only very small surfaces (a very small part of each steel ball) coming in contact with Very smooth steel plates. The result being very little friction. Once the ball diff is broken in and adjusted properly I think there can be little doubt of its advantage over the regular diff. The only trade off is higher maintainence..
As far as weight goes, in the case of the TA04 the ball diff unit weighs considerably less than the standard planetary diff.
I am sure the weight difference will vary from car to car but in most cases I think you will find the Ball diff weighs a fair bit less..