The Japanese Akita was kept as a general purpose farm dog, did a bit of guarding a bit of hunting etc. The American Akita comes from Jap Akitas brought back by G.Is after WW2 and mixed with all sorts of other breeds (GSDs, Rottis etc). The American usually has more of a solid, guardian temperament and the Japs are really weird, aloof dogs, a very primitive breed similar to the other Asian Spitz type breeds.
Personally I wouldn't own either of them, even the more solid American type are still unpredictable b*****ds that will be fine one minute and hanging off your face the next