What are some pros and cons for the different Zerg and Protoss research options?
Zerg:
Tier 1: 150 life on bunkers is MUCH better than the shrike turret. more hits b4 it goes down, starts repairing itself at half health so at 75 hp earlier, especially with increased SCV repair speed. Shrike turret is ground only and has crappy damage.
Tier 2:The fortress is much better than the perdition turret. 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. 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.
Tier 3: Neither hercules nor predator unit are really required in any mission. 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. The predator unit is very decent when combined with regenerative biosteel and science vessels, especially in scenarios with many waves of enemies. This is actually a matter of personal taste as far as I'm concerned.
Tier 4: Regenerative Bio-Steel. 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. Also, in call of the void your battle-cruisers wont even get hurt by the rift generators.
Tier 5: The zerg slower is many times better than the control tower. 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. Slow zerg are dead zerg (don't forget to use marauder slow combined with it to make the zerg sluggishly slow.
Protoss:
Tier 1: Ultra-Capacitors 5% attack speed can matter a lot on marines in a bunker. The best reason to take attacks however is if you have marauders with slowing aoe upgrade. more units slowed with marauders, more dps, and they are in a bunker anyway most of the time.
Tier 2: take the 25% gas mining. 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. Gas is often the limiting factor in the speed of your army construction. 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.
Tier 3: building 8 scv's in queue with 2 per time will rocket-launch your economy. On average a scenario will have 2-4 active gas mines in use at any time depending on your expansiveness and playing speed. those 6-12 scv's in food, combined with the advantage of not needing a command center don't weigh up against the increased production speed which will rocket-launch your campaign economy output in EVERY scenario, right from the start… If you need food later, you can always shoot your own SCV's.
Tier 4: Science Vessel vs raven is again a matter of taste, like the tier 3 Zerg upgrade. Ravens when properly micro managed can cause havoc with their damage abilities and work wonders against air units. However, if you like auto healing your tanks or anything mechanical, irradiating large groups of enemies and forming virtually indestructible death squads with neosteel battle-cruisers, I'd definitely advise the science vessels. They are my personal favorite.
Tier 5: The tech reactor is greatly superior to the orbital marine units. Although orbital drops can be useful in multiple scenarios, don't forget that with tech reactor you need only 1 of each building in any scenario, saving you at least between 500-1200 minerals and gas in most scenarios and allowing a speedy build of high tech units.
Final note:
Don't forget marauders slowing AoE upgrade in the armory. I finished the entire brutal campaign and all campaign achievements without that but it made things unnecessarily hard. Slow enemies are usually dead enemies before they even reach your base.
There are three that come to mind that I strongly feel have a better alternative.
"Build 2 SCVs at once" or "Vespene Geysers do not require SCVs for gathering"
Choose build 2 SCVs at once. Doubling the rate at which you build SCVs has an exponential impact on your economy. The 3 or 6 SCVs you lose to mining gas is negligible compared to how much faster you can get your economy going. Especially in the last mission. I completed it on Brutal, and was non-stop building SCVs for gathering minerals and repairing units. Not having to gather gas is really only a significant help in one of the campaign missions.
"Tech-reactors" or "Drop pods"
Choose Tech reactors. Not only do Tech-reactors allows you to spend fewer resources on structures, and more on units, but they also increase your unit production rate. The drop-pod ability is late in the game, and faster deployment for ONLY Barracks units is not as helpful as faster and cheaper production.
"+25% Geyser allocation" or "Instant Supply Depots"
If you are good at keeping track of your supply count, choose the +25%. While calling down Supply Depots instantly has its uses when you are trying to create a wall-in. If you can keep track of your supply count, there's no reason to miss out on +25% gas allocation.
"Weapons Upgrades grant 5% increased attack speed" or "Armor Upgrades grant 5% more HP"
This is a tricky one -- but ultimately, it really comes down to unit comp.
Ask yourself -- do you intend to invest in medic / repair tech? The longer any individual unit survives, the more it benefits from increased attack speed. Raw health only matters in the absence of SCVs, Medics, or Science Vessels. If you buy the bunker upgrades -- it makes sense to go with the weapon upgrades -- units can't benefit from increased health when they're in the bunker after all.
If a marine usually has an attack speed of .83 seconds / attack, and the weapon upgrade reduces that by 5% ( to .78 ), it takes 5 attacks for the marine to benefit from the extra attack speed (both make 4 attacks in 3 seconds, but after 4 seconds, the upgraded marine has made 5 attacks, and the unupgraded marine has made only 4 (but is about to make a fifth)
The longer you can make your units survive, the greater the benefit.
Bonus health doesn't make sense on the weak, individual units who will die quickly anyway. It, on the other hand, works better for stronger, more expensive units who are already taking several hits to down. A battlecruiser's 400 hp goes up to 420 -- which means it will survive two attacks more from a mutalisk swarm (discounting armor)
A marine would only go from 45 HP to 47.
Because of all of that, I think it's better to go with weapons speed than bonus health.
"Build 2 SCVs at once" or "Vespene Geysers do not require SCVs for gathering"
I don't agree that Build 2 SCVs at once is definitely superior to Vespene Geysers do not require SCVs for gathering. Both have unique benefits.
The Build 2 SCVs is good early game, because it gives you a faster ramp rate of mineral production early on. However, late game, this won't be so important. The Vespene Geyser research is better for late game. During late game, the player will most likely have advanced far up the tech tree. These advanced units require a lot of Vespene gas. Typically, the 2 Gas geysers at your base are not enough to keep up with the 7 to 10 mineral fields. This is where the Automated Refinery comes in. Because it does not require SCVs, it also does not require a command center. Thus, the player can build refineries at expansions, without the need to protect them since the most they could lose is a refinery (100 mineral) compared to a refinery, a command center, and a fleet of SCVs otherwise. If you are someone who likes late game, and uses high-tech units like siege tanks, wraiths, etc, you will find the Automated Refinery very useful.