The most difficult alternative

The only advantage monks have over crusaders is that they require an abbey, which you'll build anyway for the angels (because you need a seminary). Crusaders, on the other hand, need a shop, which means losing a day. If that day is critical and you have enough resources to place the angels, say, on day 5 (PowerSpot, Expert), then the choice is clearly in favor of monks. Otherwise, crusaders are much more dangerous.