Re: Aikido v.s. Hapkido
I just searched for the history of both, as a colleague of mine studies HKD.
The similiarities are astonishing:
As said before, both derive from Daito Ryu, as both founders trained under Sokaku Takeda, while I have not found witness, that both founders have met.
Both arts have been trained in their country's police and secret services in not their best times, aikido in WW II, HKD in the later years of South Korean dictatorship (I think it was by the founders direct follower as the art's head master).
O Sensei left Tokyo and retired to Iwama, the HKD master had to leave South Korea and went to the United States. For both arts these events were point of changes, when the arts grew much softer than they used to be before.
I might have messed up a little bit, but that was what I recognised.
From what I have seen, HKD seems to require much more athletic and artistic fitness from their students.
Another interesting aspect: The punches and (high) kicks were introduced to HapKiDo, when Tae Kwon Do became popular and Hap Ki Do had to prove to be a Korean Art rather than Japanese, which might be understandable after WW II and years of Japanese occupation.