The former have twice as many built-in GSP worth of supplies, so if you're skimping on resupply units they can do more before stalling (though that becomes negligible if you're bringing plenty of supplies).
This isn't true. The former formation has twice as much GSP but they use it twice as fast. In fact, the latter formation has the (small) advantage here since they require half as many supplies, thus only half as much BP dedicated to resupply units.
And they're just as (non) durable as the armored infantry against heavier weapons, which can be a big deal depending on enemy force composition.
This is actually one of the key disadvantages of armored units. If we have, for example, PWI infantry against unarmored infantry, then the extra +0.25 penetration of the PWI is wasted (along with the +1 and +0.25 GSP ton per soldier that PWI requires). Against an armored infantry or any other armored unit, the penetration of PWI becomes useful. Turning this around, armor-piercing weapons are proportionally more effective against armored units than against unarmored units.
This sounds obvious, so let me illustrate by an example: Let us compare unarmored (base 1/1 HP/armor) and armored (base 1/2) infantry units, each armed with simple PW (5 tons) for the purpose of analysis. The unarmored infantry costs 0.1 BP per unit, the armored infantry costs 0.2 BP/unit. We consider two cases, assuming equal tech levels all around:
- The attacker is armed with PW. In this case, the attacker has a 100% chance per hit to kill unarmored infantry (loss rate 0.1 BP/hit) and a 25% chance per hit to kill armored infantry (loss rate 0.05 BP/hit). Note that while it may seem like the armored infantry enjoys a 2-to-1 advantage, you can deploy only half as many armored infantry per BP and so the double number of unarmored infantry puts out 2x as many shots, therefore the loss ratio works out as even if you work through the math involved.
- The attacker is armed with PWI. In this case, the attacker still has a 100% chance per hit to kill unarmored infantry, but now has a 39% chance per hit to kill the armored infantry (loss rate 0.078 NP/hit). Even though the armored infantry can resist the weapon better than the unarmored infantry, it is not possible to "overkill" the unarmored infantry, so they perform relatively better against the armor-piercing weapon.
Of course there is the "extreme" case of, say, LAV, which as you already pointed out kills both types at 100% efficiency and renders the armored infantry a waste of BP. My point here is that even against semi-piercing weapons, armored units perform less effectively per build point than unarmored units.
The chief advantage of armored units is when you are limited by transport capacity rather than build costs, which usually means in the case of a major invasion (i.e., an alien home world) but also applies for, e.g., boarding combat or for low-tail operations like capturing spoiler race colonies. Unarmored infantry are the most cost-effective unit type, but of course easily the most transport-ineffective.
It is also worth noting that against most NPR armies, which are heavily biased towards PW infantry and CAP, the disadvantage of armor is minimal enough that other factors like evasion and breakthrough chances can become more important. Against enemies with a heavier allotment of anti-armor or mixed-use weapons, then armor becomes a more costly proposition.
The armored infantry takes twice as many rounds of incoming fire to wipe out. So back on supplies, they're placing more stress on the enemy GSP. They take half the proportionate losses in any given round, so they produce a better Cohesion Rating making them harder to Breakthrough against.
I think their longer endurance can provide better protection of high-value units, since it's not uncommon for such units to survive better and die late after the infantry has been cleared out around them, but this might be deceptive, I'd need to run the math. It's certainly sometimes true though - if the enemy isn't using Front Line Attack, their units can't hit your units behind the front line, so the armored infantry does a significantly better job of protecting a support-line stack of Light Bombardment in a trench stalemate situation.
This is true per ton, but
not per BP. Which ties back to the tonnage efficiency point as the chief advantage (the relative lower GSP use of the armored formation, depending on weapon types, is also in play but the effect is much smaller).
A lot of the nitty-gritty micro-details in practice end up depending too much on what the opponent has, and there's no way to find that out besides reconnaissance in force, so you may as well make the army you like while you're waiting.
