Moon Charm

The transformation only fires after dark, so for half of every day the slot does nothing. That matters less than it sounds, because most of the fights you would want it for start at night anyway and a boss summoned at dusk keeps the bonus for the entire attempt. Outside melee it is close to dead weight. Rangers and mages who want the look are better off waiting for the combined shells, which at least bolt something useful onto it.

Your armor stops rendering while the buff is up, though it keeps working, and any dye on those pieces tints the fur instead. Accessories stay visible. Two switches are worth knowing: put the charm in a vanity slot and you get the appearance and the growling hurt sound around the clock with none of the stats, or mark it invisible and you get the stats with none of the appearance.

Neptune's Shell wins outright. Wear both and the Merfolk form replaces the werewolf at night, taking the bonuses with it. The charm also does not stack with the accessories it crafts into, so once you have a Moon Shell the original is only worth keeping for the sprite.

Werewolves only appear on full moons, and the drop is rare enough that one night rarely finishes the job. Use a Bloody Tear on a full moon to stack a Blood Moon on top of it, and the spawn rate does most of the work for you.