Both manga are about someone from our era somehow ending up in the Sengoku era, and both are closely related to Nobunaga. In Concerto the protagonist is an ordinary high school boy who must become Nobunaga, in Chef we have a cook who takes his 21st century culinary knowledge to old Edo.
Similar in showing famous historical figures in there rise to power. Not only do both manga show well known battles in the past but also show the political aspects of war to. Both manga's conduct interesting fictional twists that allow for amazing action scenes but also funny comedic scenes as well.
Both manga have Nobunaga, his conquest and time-travel as the main focus. The main difference, however is that a real Nobunaga (who also happened to be a very ruthless and cunning, if needed) traveled back to the past, not a schoolboy. Thus, he changes history, not creates it.
Again! is about a time travel within your own life time to do something over, while Nobunaga Concerto is a comedy about a high school boy going back in time to become Nobunaga's double, these manga are very funny shounens and the humor of them is similar where they act different from how they really are. They can seem like complete fools sometimes and pretty much wise or wise guys at other times. They vary in one being modern and the other historical.