Haven't tested this, but my intuition is that the Planeswalker is going to be all around better. It has an additional ability that does a ton of damage, even at apprentice (I would happily test it for you, except I have no good way of going back to Apprentice on my Planeswalker). If you have both at the requisite spell tiers, my suggestion is to just parse it out on a dummy (10-15 minutes for each pet on an epic dummy).
