Necropolis or Castle (Which one is stronger?)

In general, humans are not more expensive than Necropolis units. Only Archmages are more expensive. Plus, you get free skeletons. So, in this regard, the factions are practically equal. The difference is that Necropolis can build a large army earlier. However, against a swarm of Archmages, Necropolis will have a hard time if that swarm appears.