Corruptor: Speed and Damage 6 speed is important just to be playable. Damage help you get the Soulharvests since his base damage is low. If you are close enough to cackle, you're in trouble already Ethereal Soldier: Reinforcement 2 + Strike or Psychic Aura 3 Reinforcement is auto - he's undead so you'll always get a ton of AP. Plus it combos with Shadestriker's illusions to get AP in the early game. Strike is pretty good - but I think Aura is better. The reason being that he'll only get to attack every 2 turns, and almost never get to double tap. Psychic Aura gives you that little bit more of passive damage every turn, which is probably more than you'll get from Strike. It also gives you a benefit without using AP.
I prefere Strike. Aura is nice and consistent damage, but Strike lets me counter some of the really obscene high defence champions. Sometime you are much better of getting his full 18 damage in on a double tap.
I'm in the same boat as Banshee regarding Strike. We have a lot of ways to get around 'bulk defenses' like RD and Arrow Eater, but we don't have too many options to get around static defense that's gotten pretty crazy (Direct LoL damage from champs is reduced by defense). At the very least, I'd run it 1 and 1. One with Strike, one with Aura. As Strike can sometimes feel hit or miss, and if you have the advantage to get two Soldiers on the board, you're likely in a really good position anyways, and can eat the lost damage from a higher aura rank. And as Nid said, reinforcement is pretty much auto. I would only hazard to say it isn't in the off chance that you are fully intended on playing full out stall attrition. And even then I'd likely take reinforcement over both Strike/Aura. Corruptor. I'm a fan of Soul Mason myself. Damage can easily net a harvest or two that may not have otherwise happened, but soul mason means you can free a slot that Tome of Hate would normally take up, meaning another finishing spell/equip you can add to your BG. But if you don't see yourself abusing your shrine all that often, it becomes a much weaker upgrade compared to the flat bonus the +Dmg gives you.
I have to basically completely disagree on Soul Mason. I can see why you're a fan of it, but it's just a "win more" kind of ability. If you get a kill with Corruptor, you're already getting ahead. So the fact that you can make that easier with the Damage upgrades makes it a way better choice, even if you don't run ToH (which has uses as a Pers Counter so you may run it even if you did take Soul Mason).
True. I don't always run Soul Mason, but if you hit a streak of running into shrine rush decks, it adds a buffer for you. But, I also forgot to mention that Soul Mason is a strong choice if someone is running alternate means to buffing the Corruptor's damage, like repurpose, Mute Stalkers, or for some odd reason Foul Rite. But yes, I can agree that +Dmg is a more consistent benefit than Mason.
