I just purchased the JConcepts spur. Its round..... Unlike the AE. They suk! I use the Losi pinions they are the quietest I have found. Use the upgraded slipper pads to. It makes the spur more stable to the center of the gear as well and absorbs a lot of noise as well. It takes away from the cooling vents in the JC spur though.

I put all my shims in the tranny on the gear side. You'll be shocked at first but a after run it loosens up and you will love how much longer the diff gears last. And its quieter. Smoother acceleration gear to gear.