A ball diff will always be freer than a gear diff as the gears have to run on shafts and will have friction due to the gear bearing on the shaft. The shafts will 'twist' in the plastic housing due to the loads put on them by a heavy car, a lot of power and quite a bit of grip, especially on grass and astro. and, lastly, there is the friction from four gears imperfectly made rubbing each other.
However, because the ball diff will not be that free for that long (See post above from Xracer) it should be possible to make one feel like the other. The tighter the diff is, the more the tail of the car will slide - drift cars have locked diffs to make then slide! - so the more grip the track has the tighter the diff can run. That's one of the problems with a ball diff, it isn't that adjustable for grippy tracks.
It's not my class, but when I did run off road I wish the manufacturing capabilities could have produced a decent gear diff. If I did go back into it I would certainly try one over a ball diff. HTH
