From a purely PC balance perspective I'd venture the Necromancy army is slightly more powerful but the leadership follower army is significantly more flexible.
Looking at it from a less game oriented aspect the army o' zombies or skeletons have a significant logistics saving over a humanoid army as they don't need a baggage train, healers, etc.
However zombie armies are slow moving, largely incapable of sophisticated tactics, vulnerable to collected cleric turning, etc.
In contrast given the financial resources of a powerful lord humanoid warriors can be well armed and be given horses to increase thier mobility edge. If your army is largely heavy cavalry it has a significant power mobility advantage over the undead army.
For the most part zombies and skeletons provide a useful meat shield (not that humanoid warriors don't) for the stronger undead within your army. Wights, Ghasts, etc begin to give significant advantages over most human troops and are generally self-replenishing resources.