Dople blades are nearly worthless. You get a marginally higher atk and are then forced to spec alot more agi to get good aspd. Any and all skills that effect/require swords can also effect/require daggers, whereas there are dagger only skills such as double strike. Also, many more classes can use a dagger than can use a sword.
I would totally recommend a +10 DD mg to a new player as their first mvp-carded weapon.