Tier 3: building 8 scv's in queue with 2 per time will rocket-launch your economy. If you plan the supplies right and micro manage just a bit more orbital supply depots hardly speed up your production capabilities or game-play in comparison. Gas is often the limiting factor in the speed of your army construction. Being able to produce earlier in the scenario can save you troops to help with the next waves, and cumulatively save you a lot of hassle. more units slowed with marauders, more dps, and they are in a bunker anyway most of the time. The best reason to take attacks however is if you have marauders with slowing aoe upgrade. Tier 1: Ultra-Capacitors 5% attack speed can matter a lot on marines in a bunker. Slow zerg are dead zerg (don't forget to use marauder slow combined with it to make the zerg sluggishly slow. Although the control tower is manually targeted and can demolish enemy waves by controlling an ultralisk in that wave, and when properly used can even be used to build a second army with separate food supply, in the end the slower allows you to move down any zerg units with them doing little or no damage to your base, even on brutal. Tier 5: The zerg slower is many times better than the control tower. Also, in call of the void your battle-cruisers wont even get hurt by the rift generators. The mana upgrades and starting energy increase are very useful in many scenarios but in the end the regenerative bio steel will allow you to sweep any map on any difficulty with more ease than normally. This is actually a matter of personal taste as far as I'm concerned. The predator unit is very decent when combined with regenerative biosteel and science vessels, especially in scenarios with many waves of enemies. Normal drop ships will work fine and even heal nearby units, despite the incredible amount of units a hercules can travel it requires a special building to be built and you won't have your first one until you have spent hundreds of minerals and gas. Tier 3: Neither hercules nor predator unit are really required in any mission.
The fortress upgrade is only really good in a few scenario's but packs a decent punch, is repairable, and adds hp's to your building, allowing you to use it as a wall to slow down enemy advancement, including on the last map. The perdition turret isn't better than a bunker with 4 marines and a firebat, gets destroyed often, and costs precious gas which you need for units and upgrades.
Tier 2:The fortress is much better than the perdition turret.
Shrike turret is ground only and has crappy damage. more hits b4 it goes down, starts repairing itself at half health so at 75 hp earlier, especially with increased SCV repair speed. Tier 1: 150 life on bunkers is MUCH better than the shrike turret.