The Importance of Upgrades
From Liquipedia StarCraft Brood War Wiki
Upgrades are one way to make your units more effective than your opponent's units. Remember that the more units you have, the more cost-effective and important upgrades become.
Generally, attack upgrades are more important because of the fact that attack upgrades often add more than +1 damage, whereas armor upgrades always remove one point of damage (with the exception of units with multiple hits in their attack, discussed below). Shield upgrades are rarely invested in since shields take full damage from every type of attack.
However, there are exceptional cases where upgrading armor would be more beneficial than upgrading attack. In one case, an armor upgrade would increase the amount of hits an opposing unit (usually one with relatively high cooldown) has to land in order to kill a unit. Another case involves units that deal splash damage. If armor is upgraded, every unit in the splash vicinity benefits from the armor upgrade, resulting in a large net reduction of damage.
- For units that get more than +1 damage per attack upgrade or units that deal splash damage, it is nearly always better to upgrade attack before armor.
- For units that often receive splash damage (such as Mutalisks against Corsairs, Valkyries, or other Mutalisks), it is almost always better to upgrade armor before attack.
- Some units in the game have their damage split up in to multiple hits. This affects the way that armor upgrades work out - target armor will be subtracted once per attack.
- While playing Protoss, it is standard practice to get the Protoss armor upgrade before the Protoss shield upgrade. Most of your Protoss units have more health points than shields. The Protoss armor upgrade costs less money and less time than the shield upgrade to get.
- Shield upgrades, although they apply to every Protoss unit and building, are not worth investing in unless you have a significant amount of spare resources.
This match-up is actually the mirror where upgrades are the least important, just because of the sheer Hit Points that the units have. In general, most players tend to get their +1 Attack upgrade after they secure their second (or even third, depending on the map) expansion. From there, you see upgrades continuing, with players frequently switching between upgrading weapons and armor or going for a 2nd Forge once the first attack upgrade is done.
This match-up usually has the least number of upgrades, just because most games don't last long enough to upgrade. However, if you do get upgrades, you should get air armor upgrades, because it reduces the damage that your opponent's Mutalisks would do, and has great effects against the Mutalisk bounce damage. As a more in depth analysis, consider the following: due to a its bounce damage, a Mutalisk normally does 13 (9 + 3 + 1) damage. With an attack upgrade, a Mutalisk would do 14 4/9 ( 10 + 3 1/3 + 1 1/9 ), an increase of 1 and 4/9 damage. Also due to the bounce damage, an armor upgrade would reduce damage by a total of 3. Thus an armor upgrade would be more beneficial than an attack upgrade. The only time that you would get attack upgrades would be if you're trying to snipe drones or sunken colonies. Some players prefer to upgrade +1 Air Attack due to its cheaper cost and faster upgrade time. Also crucial to this matchup is the Metabolic Boost upgrade, while some players opt out of it in favor of faster/more Mutalisks, it leaves their Zerglings weaker in an engagement as they are less mobile, and it leaves them vulnerable to a runby as Speedlings can easily harass a mineral line with impunity against unupgraded Zerglings. It is viable to skip the speed upgrade if your Zerglings have a ramp to hold.
Weapons upgrades are very important in TvT, as they allow you to kill off an opposing Siege Tank in two shots, instead of three. This ability allows you to break tank contains much more easily, and also gives you an advantage when trying to blitz through a tank line. Since defensive upgrades cannot prevent your Tanks from dying to sieged Tanks in two shots (once two attack upgrades are done), armor upgrades are rarely seen.
Once the Terrans transition into a lategame combination of Wraiths and Tanks or towards Battlecruisers, they usually start upgrading from two Armories as quickly as possible.
In this match-up, upgrades are very important. The Forge first build allows you to start the +1 Attack upgrade immediately, which will help to be ahead of the Zerg in weapon upgrades the whole game. If the Protoss is ahead of the Zerg player in upgrades, Zealots will kill Zerglings in 2 hits, and kill Hydralisks in 5 hits, instead of 3 and 6 hits respectively. Zerg players have tried to counter this by building 3 Evolution Chambers, and start moving out when they have an equal number of upgrades.
Also, upgrades for air attack can be very beneficial when going Corsair/Reaver or Corsair/DT, because of the sheer amount of additional damage that the Air Upgrade can provide for Corsairs. "Critical mass" for Corsairs(the minimum number of Corsairs needed for them to maintain air superiority) decreases from 6 to 5 Corsairs with the +1 Air Weapons Upgrade.
Other than getting Carapace to keep up with the attack upgrade level of your Protoss opponent, most of the other upgrades tend to have less priority, however you need attack upgrades desperately once the Protoss' level of armor upgrades rises. Overlord speed upgrade is extremely useful in order to counter DTs effectively and escape Corsairs. Hydralisk Speed and Range are also essential in being able to use Hydras in your army, as well as to ward off Reavers and Corsairs. Towards the endgame, an important upgrade to research is the Adrenal Glands upgrade for Zerglings, which reduces their cooldown and significantly (by 1/3) increases their damage output.
There are several important upgrades in this match-up. Most notably, you should try to start melee upgrades at about the same time as you get your third Gas and/or Hive Tech, depending on what build you're going to choose to go. This is because you want to be able to kill Marines with either Lurkers or Zerglings as fast as possible, and upgraded Zerglings, Lurkers, and Dark Swarm do a very good job of defeating the Terran ball.
During late game, you need all the armor upgrades possible to reach the groups of Marines and Medics with your melee units. Ultralisks that have all 5 additional points of defense are very difficult to kill with infantry.
If the Terran chooses to go mech against you, you should try to upgrade as fast as possible, so that your units take even less damage. The reason is that upgrades for Terran do a large amount of additional damage, and your additional armor and weapons can negate that advantage.
If you intend to go for a Hydralisk/Lurker build, or any Lurker-heavy build, missile upgrades are important. This means Marines will die in two Lurker spines rather than three.
The most important upgrade in this match-up is choosing between +1 Armor and +1 Attack for your Marines. +1 Attack, with stim and range are very good at defending against Mutalisks. Even without +1 armour upgrade researched, your marines should stay alive with proper medic use while assaulting sunken colonies. However, +1 armor allows your Marines to live through an additional Lurker attack, as well as an additional Sunken hit. However, bear in mind that if you use the Stimpack ability before a fight(as you should) your Marines will have reduced hit points and thus die in two Lurker spines anyways! The most common upgrade researched first is the +1 Attack Upgrade.
Most Terrans opt to get a second Engineering Bay when their first upgrade finishes. This is especially important for SK Terran.
Getting upgrades for Terran is key, because it allows you to take much less damage from both Dragoons and Zealots, as well as doing much more damage. Tanks get +5 damage (in Siege Mode) from each upgrade, and Goliaths get +4 air attack. This large increase in damage allows the Flash style of upgrading to work.
Protoss upgrades in this particular match-up aren't that important initially. You should focus on getting Tech out (either Arbiters or Carriers), which will allow you to be more mobile than the Terran, and abuse that mobility to be able to attack wherever the Terran isn't.
Once your Tech is done, you have to catch up in upgrades. Since Zealots have multiple attacks, their effectiveness against armor-upgraded Terran units diminishes severely if they do not receive attack upgrades. Attack takes priority, since sieged Tanks are hardly reduced in effectiveness through armor upgrades.
When transitioning to Carriers, both kinds of upgrades are vital, since Goliaths have a dual attack, and each single Interceptor profits from the attack upgrade. Most players start their first Air Attack upgrade as soon as they decide to switch to Carriers, way before the first Carrier even warps in. It's the only way to bother terran at all, if he made mech upgrades (attack + armor) and has 7+ factories. Upgrades on both sides (attack + armor for Terran, attack for Protoss) have enormous importance in Goliath - Carrier duel.
Armor should have priority over shields since Terrans who use EMP can negate any shield upgrades you have in the late game.