What's the difference between Usui and Jikiden Reiki? Should I learn Jikiden Reiki if I am already an Usui Reiki Master?

Since Mikao Usui created his Reiki method, all styles of Reiki ultimately have Usui-sensei as a part of their lineage. That however, does not mean that all Reiki styles are the same. 

What makes Jikiden Reiki different is that Jikiden Reiki has remained preserved in Japan in its original tradition for all these years. It was passed on directly and unmodified from Dr. Hayashi, one of Usui-sensei's students, to Chiyoko Yamaguchi, who later on, established the Jikiden Reiki Institute in Japan, which is dedicated to preserving the original Reiki tradition. Dr. Hayashi also taught Reiki to Mrs. Takata in Hawaii, and it was Mrs. Takata's students and their students, who decades later developed new Reiki styles, including Usui Reiki. 

Naturally, because of language and cultural differences, Reiki has changed quite a bit outside of Japan. Many concepts have been added and removed overtime. So the main difference between Usui Reiki and Jikiden Reiki is essentially that Usui Reiki is more westernized and Jikiden Reiki is more traditional and more Japanese. There are lots of other differences, such as the symbols used in each style, the actual Reiki techniques, and so on. 

When comparing Jikiden and Usui Reiki styles, it is important to remember that there is no right or wrong way, but the two are just different.

I am often asked by Usui Reiki Masters about Jikiden Reiki. Many of them are wondering about why would someone, who is already a Reiki Master of Usui Reiki would want to take a Reiki class in a different style. Well, there are two ways of looking at it. Of course there is no need to take a class if you have no interest in it. On the other hand, if you are curious about the Japanese Reiki tradition, preserved virtually unmodified from the original way Usui sensei used to practice it, then you may find Jikiden Reiki very informative and interesting. If you are interested in learning techniques that have disappeared in all westernized schools of Reiki, then Jikiden Reiki training may be of interest to you. 

In general, I think that learning something new is never a bad thing :) (only if you are drawn to it, of course). So if learning Jikiden Reiki sounds like a waste to you, then I would not recommend investing you energy, time, and money into it. If learning Jikiden Reiki sounds exciting, then I would certainly recommend taking the class and learning Reiki in its original tradition.