I personally think creatures are better than spells because a creature could do as much as a spell, could cost the same and could make you discard the same amout but a creature do as much damage as a spell every turn. (untill it's killed that is)
Picture it this way. Would you rather cast a Fluffy or a Draught Of Living Death?    A fluffy costs the same, and makes you discard the same amount but a Fluffy does this every turn!
Then you could also cast a Steel Claw that'ld do the damage again and also have a Hagrid out that'ld make it do 2 more damage than the best spell every turn!