I'm happy to see your good R/C karma coming around to say hi for you!
Are you going to build carpet or dirt racers if you end up with both?
Your carpet car... new guys will tell you that you want the gear diff for carpet. A gear diff will take the shock loads from a brushless on carpet "better" than a ball diff.
The balls can and do wear your diff parts as they operate but on high traction, if the slipper isn't set pretty much perfectly, the extra toque load is transferred through the ball diff and it also acts as a slipper.
You'll fry the diff if you let the ball diff slip too much.
The nice thing about a ball diff is that when its set correctly, it behaves as a "posi", meaning both left and right wheels have "locked" together in forward motion. This is extra benneficial on loose surfaces.

Plan on stocking exta parts for inspection and rebuilding often.
No oils in ball diffs. Ball diffs are usually very easy to adjust by removing a dogbone to access an adjustment screw.
Your gear diff either takes the load and the car goes forward or you strip gears out. There's no "give" or slip with gears.
A gear diff, by design, can't "lock up" like a ball diff "posi".
In a perfectly straight line, the gears are seeing a perfectly even load (in a perfect world). Any turn means one side and the other side of that diff IS seeing more+less power with gears.
If you over power one side or the other, your car will loose traction on one side or "diff out". Oil in the diff case will help slow the rotation of the gears and help prevent "diff out".
Super heavy oils can get you "posi" effects also.
Plan on stocking different weight oils to "tune" your cars diff to different track conditions or possibly having a couple built gear diffs in your toolbox on race day
If your slipper is set correctly, your diff won't ever see the "breaking" shock load on throttle. Your diffs should be fine, ball or gear, IN THEORY.
Again, my car is very different than your BUT I ran carpet ONLY with my ball diff Mid for years.
I am old and have hearing problems sooooo...

Actually, I'm going to try a gear diff with a one-way diff up front and an actual slipper.
I think I'm going to hate it and I'll have a ball diff in it asap... we'll see.
My car is 4x4 and a belt anyways so its different.
I don't know if the Associated parts are availible and how expensive it would be to change over.
My personal experience with OTHER CARS is that I would grab up both those Associated (if they are in good shape, sounds like a good deal) and I would put one together and run it!
Unless the parts supply dries up, those cars should do fine for you!