Both are viable for legend, though which one you pick should be reflected in the rest of the decklist. If you are using Justicar, for example, you should cut some of the whirlwind based removal like Sleep with the Fishes because you don't have the DK hero power. The DK variant is obviously a lot better vs paladin while the Justicar version is slightly better against most other things. Personally I prefer Justicar because it's less clunky to empty from your hand and there's matchups where you don't want to play the DK but are still forced to because of Dead man's hand
Dunno about Baku. She's more of an attrition warrior card, not mill warrior. How do you mill the opponent when you have no means of extending your deck size?