Not that much of a difference as Gravebone is a Mage that has support damage. 1 damage difference requires 1cc to activate means Chimera needs 5 resources to become 5/2 while Bella is 4/2 + 1 card.
Actually, in practice, I find myself casting more DeathKnight than Bella as it also has card advantage. My main reason for Bella is the card art and it's a high damage cc that fits to portal deck
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