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For me, the difficult choice is between Angels and Champions. Of course, Angels are useful because they can resurrect a hero or an army (although in Equilibrium, mages now have resurrection, which slightly reduces the value of Angels). On the other hand, Champions are very cheap, and they deal a lot of damage and have the first strike. However, I usually choose Angels, as the hero's life is more important, and a resurrection is never a bad thing...
In general, Champions should be compared to Thunderbirds (they have the same stat increase, so theoretically, they should be equal units), but Champions are significantly inferior... And Champions die quite quickly; the first strike doesn't always save them....

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!