Who else can effectively "choke" champions with low-level units, if not the Nature faction? In my opinion, no one. Other races simply don't have access to that many low-level units. And if that's the case, then it turns out that the only weakness of champions is their vulnerability to druids? I'm not trying to "defend" them; these units are quite mediocre, but it's worth noting that their ability to quickly gain experience, combined with their first strike, is a serious cause for concern.

By using two types of level 3 units at once, you sacrifice one hero...which would definitely be useful. And besides, you reduce the attack power of both resulting stacks. But since you mentioned a large number of castles, you probably meant playing against the AI? If so, then any choice will work, so what's there to discuss?