I see armor as a the most reliable defense. I know exactly what I paid for, and it gives an exact amount of protection. For instance my TEC ships for the most part have 50% more armor than hull, giving the 150% more health than hull alone.
Shields give you a chance, which as it degraded lowers your chances to survive. With 4 shield your are 50% harder to kill, but the 50% of shots that get though will eventually force a systems check and perhaps lower your shields performance. This mean in the end you won't get as much out of your shield when some real heavy fire comedy your way.
Same goes with ECM, I think is 2 ECM for the 50%...but like shields they can degrade as you take fire. However if you want to pay a bit more stealth doesn't degrade, making it s better on paper choice.
When we were testing we ran into this debate, as while my ships lacked ECM or shields thier armor made the stipidly hard to kill, while my opponents shields and ECM were worn down by my ACR weapons and fire control.
These things change when you ad stuff in like reinforced systems, where its much harder to knock out systems.
