Armor reduces all damage. Resists reduce specific damage. Calculations I've read, do the math as shown:
Total Damage * (Armor Reduction) * (Resist Reduction) = Damage Taken
So for instance, you had armor equaling 60% reduction, and fire resist equaling 50% reduction and you were going to be hit for 20,000 fire damage. The math would be:
From Armor reduction: 20,000 * .40 =8,000
From Resist reduction: 10,000 * .50 = 4,000
So your total damage taken would only be 4,000.
So Physical Resist would be in the second calculation, but only against Physical damage. Whereas the armor reduced damage regardless of the source.
To answer your last question though, All Resistances is preferable unless you are focusing on a specific struggles you may have. For myself Plagued, Molten, and Wallers with Mortar are the most difficult. So I prefer higher poison and fire resistance if available rather than resist all.
Not sure if I should balance the two or just go for elemental?
Cheers.
Why? Why not ele or all
Total Damage * (Armor Reduction) * (Resist Reduction) = Damage Taken
So for instance, you had armor equaling 60% reduction, and fire resist equaling 50% reduction and you were going to be hit for 20,000 fire damage. The math would be:
From Armor reduction: 20,000 * .40 =8,000
From Resist reduction: 10,000 * .50 = 4,000
So your total damage taken would only be 4,000.
So Physical Resist would be in the second calculation, but only against Physical damage. Whereas the armor reduced damage regardless of the source.
To answer your last question though, All Resistances is preferable unless you are focusing on a specific struggles you may have. For myself Plagued, Molten, and Wallers with Mortar are the most difficult. So I prefer higher poison and fire resistance if available rather than resist all.
you need them all
Also, on rares, you can get dual resist items
+all resists
+X resist
+other stats
you get the point. ALL resists are necessary though