At first, let us look at the basic numbers:
- 80 Shield
- 80 Health
- 1 Armor
Every Armor upgrade gives them +1 additional armor and every shield upgrade +1 shield armor. But how does Armor work exactly? Liquidpedia tells us:
Damage by incoming attacks is reduced by the amount of armor: every point of armor reduces the damage received by one point. Units attacking in volleys, like the Zealot and the Thor, have the target's armor applied to each of their attacks. [...] Protoss Plasma Shields use a separate armor value for computing damage to the shields, starting at 0 for all Protoss units and buildings and growing by 1 for every Shields Upgrades.
During early stages of the game it is pretty easy to micro away Stalkers that took too much damage to their shields and prevent damage to their health. Shield upgrades would have an advantage there. But as we are talking about 2 upgrades it is not early game anymore and army sizes are getting bigger.
Considering Stalkers amount of Health and Shield is the same, damage reduction with both upgrade combinations is equal. That is why we want to look at how many
hits a Stalker can take from opposing units. The spreadsheet indicates that +2 Armor has a slight advantage in general.
Additionally to that you have to be beware of
EMP in PvT: If the opponent has Ghosts, Shield upgrades may be worth much less.