[Endoperez]

The manual lists some probability numbers. Mind Burn was probably 10 against magic resistance, with (virtual) open ended dice rolls on both sides. With magic resistance 24, the chance of beating it would beabout 1%. The chance to beat mag res 18 would be just 6%.

Units with low hitpoints are pretty easy to kill.

Soul Slay does work against tough guys... but only when massed. A single Soul Slay COULD kill even your pretender, but it won't - well, except when you've almost finished your enemy already and mr Murphy decides to pay you a visit.

Ether Lords and Angelic Choir are both very expensive in pearls. The first one gives you Astral 3/Death 2/Random 1 mage, which can be nice, and giant-sized, ethereal creatures that devastate magic beings. They are good against armies with lots of summoned units (all summoned units aren't magic beings, but the better ones often are). Etherealness and good protection are VERY rare combination, and very effective as well.

Heavenly Choir gives just one commander ATM. If all 13 were commanders, as the manual implies, it'd give insane amount of flying priestly power, and would give you three Air 3 mages in addition to the Seraph. However, at the moment only the Seraph is a commander. He's a 77-hp, Fire 4, Astral 4, Air 4, Holy 4 commander who can fly 7 provinces at once and can destroy armies in melee without any equipment, but 144 pearls is still a steep price for a single commander.

Golems are made used more often than either of those, and Astral gems have many other uses. I presume your question went unanswered this long because most players wouldn't choose between those two spells, having used the gems for something else entirely.