Diff gears

[jcb]

As my S4 uses the same transmission as the BJ4 Worlds I have the following most likely daft question:

Whats the difference between using the normal TC3 and TC4 diff gears compared to the NTC3 diff gear?

I know the nitro one has a plastic (sort of) bushing to fill out the space in the centre where the bearing goes, but beyond that they have the same number of teeth so why use the TC3/4 gear instead of the NTC3 one, and does it affect the handling?

Hope that all makes sense.