Which upgrade is best among these 3 options for CastleStorm?

Which upgrade is best among these 3 options for CastleStorm?

I suck at strategy games. I have heard that those who are good at strategy
games are able to take great decisions by noting down various parameters
in the game and then calculating the best upgrade to take. For ex. the
time taken to build a unit, the cost of the unit, the upgrade cost, the
amount of upgrade that is given and then based on all these factors then
decide which is the best upgrade to take.
So I'm trying to calculate my next purchase in the game CasteStorm that
I'm currently playing.

A normal soldier unit costs: 10 resource (or approx 3 seconds), has 10
hitpoints, does 10 damage. Upgrade cost: 2000
After upgrade: cost: 10 resource, has 12 hitpoints, does 12 damage.

An Archer unit costs: 20 resource (or approx 6 seconds), has 24 hitpoints,
does 8 arrow damage. Upgrade cost: 2000
After upgrade: cost: 20 resource, has 30 hitpoints, does 10 arrow damage.
Heavenly swords magic item cost: 40 seconds, does 75 damage.
Upgrade cost: 2250
After upgrade: cost: 40 seconds, does 85 damage.
Now considering the above parameters how can we calculate which upgrade is
best? Let me know in case you guys need any additional parameters.