Necropolis discussions.

In a quick game, you can certainly upgrade the Dark faction, but it's unlikely you'll be able to fully upgrade the guild! Summoning will be more effective, especially since you might be able to build the guild to the end! You might get a Phoenix, which would be great.

90% of players are clearly fixated on the Phoenix, but it's not always that great:
1. It requires 35 mana, meaning in many cases it can only be summoned once (we're talking about a quick game).
2. Even if you ban the Engineers, you're not immune to the Rapid Fire Ballista and Fire Arrows, which can take down the Phoenix or the entire army in 2-3 turns.
3. Opponent mages can easily have Deadly Cold or Chaos, which can wipe out the army while the Phoenix is being summoned and takes 2 turns to fly in.

Summoning can be a good upgrade for the Mines, Hive, Elementals (if you have a rough idea of the terrain where the battle will take place), etc.
But in many cases, the combination of Mass Plague and Unholy Word is clearly better. 2 basic Curses with 15 Witchcraft and Pain Lord = 744 damage for 30 mana to all enemy stacks. And how many armies can survive that for 3 weeks? And if something does remain, it's immediately visible and you can weaken it with troops from the start. Even against Inferno, Darkness works, as Mass Curse and Weakness turn demons into weaklings; only the potential Ballista and Imps pose a threat, but they are even worse for Summoning.
By the way, emergency guild upgrades sometimes happen, but it's more of an exception. High-level spells are obtained when you take the corresponding buildings, but they might not give you what you need. And if you're building an army, the guild will be at most level 2-3, but that's enough for Plague.