While the druid does have more corrode spells than a wizard, you still only get so many spells as you level up and when playing a mutliclass those spell choices have to be balanced against good caster passives (combat focus, fast casting, increased range, etc) and good paladin actives and passives - so in the end you probably would end up taking about the same number of spells no matter which one you pick.
I would suggest mapping out each choice and see which feels better to you. Druids tend to have better AoE options and some decent healing, if you don't see that as being to 'light' for a dark knight. Wizards will get better self defense spells - your knight will be a lot more tanky.