From a modified racing perspective, Xenon spur gears are my favorite. I find the diff is more consistent from the beginning to the end of the run with them. The octogon shape diff ball cavities locate the diff ball correctly, so they are all solidly in a line. While they generally give a more free spinning diff, the circular cavities allow for a little more diff ball wandering, so you may have diff balls on a different PCD. This can decrease the life of the diff balls and diff rings, and make the diff perform inconsistently. It may not be as free, but consistent performance is more important to me.

I also like Xenon spur gears a little better than some others because of the larger root thickness. This keeps the gear aligned properly. Some others have excessive float side to side since they are thinner than 1/8". This allows the gear to tilt/twist at any given time when you apply power or brakes quickly, increasing the risk of abnormal wear or damage.

-Korey