9EK is the factory plug. The 8EK is, as Jaysin says, slightly hotter. It will tend to foul less, although this bike isn't prone to fouling to begin with. "Hotter" does not mean a hotter spark, but rather the electrode has less insulation on it so the plug itself maintains a higher temperature. This has advantages and disadvantages. The disadvantage is they plug tends to wear faster and there's a higher chance of preignition due to hot spots on the plug. The advantage is less carbon on the electrode and therefore a more reliable spark.
That said, this engine doesn't really have any of these problems. Either plug will work well.