It'd be the best if both towns were released, but if I were to choose one from those you mentioned, I'd choose Kronverk. I like the concept of town of dwarves. Plus it reminds me about "Hammers of Fate" addon (for Heroes V) atmosphere.
Cathedral is interesting too, but for me a fine mix of dark powers and fallen knights can be done after Kronverk. I'm not saying that Cathedral is worse, no - it might be even better if it's about gameplay and fitting Antagarich itself. It's just... I like Kronverk more as something fresh in Heroes III