Cognac is sweeter than the rye whiskey (or even bourbon) on which the Manhattan is classically based. Cognac is also usually slightly lower in alcohol...
This was an excellent drink! I made it exactly as written except for the bitters. I've just bought a bottle of Bittermen's Spiced Cranberry bitters, saw those were recommended for Cognac/brandy drinks, and subbed it for Angostura. Delicious!