Which creatures from the alternative upgrade options do you choose?

I wouldn't say that shamans are completely useless. Considering that they slow down/speed up by 30%, the effect can be quite significant. I always cast it on centaurs first, then on executioners/chieftains, and then depending on the situation. Maybe sacrifice and cast magic again, or maybe send them into battle. There are also situations where some archer needs to go before the centaurs. You cast the slow spell on these archers, and the centaurs attack first (of course, if the initiative difference is small). I'm more inclined towards Daughters of the Earth. However, I don't hire them at the beginning of the game, as it's too expensive; they are more useful from level 5.