Nihonichi Momotarou

Both anime adapts the story of Momotarou. While Nihonichi Momotarou keeps the original idea, Momo Kyun Sword remodels the plot completly different. Both are fantasy. 

Both anime adapts the legend of Momotarou. While Nihonichi Momotarou keeps the original plot, Momotarou no Umiwashi is more of a World War II propaganda. Both are short, old, black and white movies. 

Both are short movies adapting well known Japanese folk tales. Urashima Tarou adapts adapts Urashima Tarou legend, Nihonichi Momotarou adapts legend of Momotarou (obviously). Both are black and white anime from 1930 era. 

Both series utilize old Japanese folklore to tell stories of heroes. Hoozuki no Reitetsu utilizes the same story and characters, and Nihonichi Momotarou tells the story of Momotarou that is only alluded to in Hoozuki no Reitetsu's early episodes and cast throughout. Highly recommend one to the other. Practically canon.