Are shield upgrades more effective than armor upgrades?

Most protoss units have more health than shields, so armour is worth more than shields in terms of extra health. This is compounded with the base armour of the units. It is much better to have armour 2 and shield 0 than armour 1 and shield 1 in terms of how many hits it takes to kill you, when you are fighting units such as zerglings and marines. This means armour increases the chances of your units being left alive to regenerate shields!

If you are going air, or archon you might prefer to get shield than armour. Shields will also affect your buildings if you are going for a lot of cannons.

Ok, both have their place and their time. There is no "which is better. I want to only get the one that't always better". While I'd agree with that statement, get the better upgrade because that would help you win ... it is still flawed because it asks the wrong question.

Instead ask WHEN is EITHER better than the OTHER ?

So here we go. We start by analyzing both of them: Excluding the infamous archon, no protoss unit has more shields than health, and immortals only benefit from shield upgrades against units that do less than 9 damage. What does this mean ? having +1 armor will help more in a single fight because your units will benefit more from the upgrade because more health, means more hits, means more damage is negated.

BUT Shields are this magic thing that regenerates really fast out of combat. Also it is universal ... from buildings to gateway units, robo units and even flying units.

So why should you care ? well you need to ask yourself: What do I need. Do I always fight whenever I am guaranteed to do damage and run away (not trying to win the game, just get ahead of the competition) ? am I making both fliers and ground units ? If you answer yes to 2/3 then you want shields

In every other scenario, you want armor.

Also there is the possibility you have a lot of extra gass for some reason ? go 3xforge :P