My rule of thumb is ball diff everywhere LOL
Seriously though so long as you can get all the forward drive you need from a ball diff it will always be safer in feel than a gear diff.
Only place I would consider running a gear diff is ultra high grip (say Silverstone indoor) where you do have to nip up the ball diff to be sure of getting all the drive available which does dull the diff feel a little and increase wear rates.
The trick to ball diffs is;
1) use the right greases, especially on the main balls. I have found none better than the Xgear XGS153 (now listed as K.96506) available from all good Kyosho stockest.
2) good quality balls, I'll now only use either genuine Kyosho or RudeBits carbide balls. There are other really good ones out there but those are my preferred ones.
3) the initial tightness on build - best way I've found is to lock the two outdrives together and set the tightness so that when gripped between finger and thumb you can just (with real effort) force it to slip slightly - don't force it too much!
4) Break in - I run 2mins per side with the other wheel off the ground (hold arms to make driveshafts straight to reduce vibration feedback during break in) then swap to the other side and repeat. Normally find that using max throttle trim on my Sanwa radio is about right, something like 20-25% throttle is enough.
5) The slip test - after break in is complete I check the diff by putting a drive into the screw head and turning the opposite wheel between finger and thumb with a relatively light grip until you feel the rotation go dead - the same feel as you get when you spin a nut down a clean bolt and it just bottoms out. Don't over do it. I then add about 10 degrees extra.
6) Recheck after each of the first 5 runs if it was with all new components, 2 or 3 runs if reusing components.
7) When stripping the diff down always sand the main plates, even if they look ok, until an even dull shade in the ball contact areas. I tend to use 2000 grit and motor cleaner to wet the paper and moving the plates over the paper in figure of eights turning the plate through 60 degrees after about 10 figure 8s.
If you find that after two complete rotations of the plates using the above method that you still don't have nice dull surfaces and/or ball tracks still showing it's time for new plates.
Tune to suit driving style from there.
Perfect ball diffs every time
