Targeting priority is somewhat situational or random as well. I had one game where I played an Hourglass and a Pulvy on the same turn, because I needed the pulvy's destroy ability to keep me in the game. I figured the hourglass would keep it safe, but the AI shocked me by destroying the pulverizer instead of the hourglass, and I lost as a direct result of the AI's decision.
I had another game more recently where I had two SoGs and a Gravity Shield in play. I had just seen this thread posted, and so it was fresh in my mind. I watched the AI target my SoG first, and then later on the same turn target Gravity Shield, rather than the remaining SoG. I had no other permanents except quantum towers.
I don't remember the exact spells used, but maybe targeting priority is different depending on the method of targeting. (explosion/deflag, steal/imp. steal, pulverizer, butterfly effect, mutant with destroy/steal)
