Sorry didn't know that InitialD is the only one allowed to use smilies.
And for my opinion about the FPS and standard balldiff:
If you want the best and easiest way to a adjust your diff use an FPS, it's will always keep the same setting during a race. But because of the weight the diff needs more power from the engine to start turning so its slower to drive.
If you you want to go fast, use the standard balldiff with diff adjuster.
The standard balldiff takes alot of time to adjust. But a great option to counter this is to use the External Diff Adjuster. If you use this one; you need to polish the outside of the dif where the external dif adjuster turns (see pic)
But than again you didn't ask my advise.