Orcs. Upgrade or alternative upgrade?

It depends on the situation. When besieging a powerful city of Cyclops, use Free creatures. If you have a weak hero-mage, use Goblin Warlocks; Centaurs are good for combat (you can also place Warmongers nearby); always take Daughters of Earth; with Wyverns, there's a trade-off, as some have poison and regeneration, while others are scavengers.