The untis you recieve are random. That said, I spent some time seeing all the diferent untis that can rewarded for the Random Gaurdian Unit choice. I have seen up to 5 units spawn per pick, but these untits are generally inferior to the single untis that spawn, but not allways.
I play on ridiculous & the following 2 untis are what I aim for when I do not want to / or simply can't afford to leave a hero behind for city defense.
At city lvl 2 the best choice I found was the Thug, 8 damage with 3 armor & no retalition for its attacks.
At city lvl 3 the best choice I found was the Young Ogre, 16 damage with 6 armor.
Those 2 units make your capitol vitually untouchable by the npc's that spawn & stack up early to mid game. So long as you fight smart in tactical combat versus very large stacks
Sometimes I may take a unit at city lvl 4, but most often at that point I go for Tech or Gold.
Anyways good luck and happy hunting.
