I'm torn between Ironman and TaN. First of all I make some assumptions.
1. 5 ways to heal to 100% and 3 ways to heal less than 100% after dlvl 4. I think the 5 ways occur more regularly, so I assume most/all heals are full ones.
2. To get 99% efficiency from TaN between heals, you need to be reduced to 2% health. This is quite risky, whereas healing with Ironman always gives an extra 10, 20 or 30 hp on top of what you have.
3. When in danger, my armor will always be at least [2]. This could be Blue armor, or damaged Red armor. Armor has been quite plentiful for me in 0.985. This is probably actually the best condition for TaN, so all my calculations have been made with this value. Having less or more than 2 armor can make TaN less valuable as explained below.
4. Almost forgot this one, but I'm assuming you're being attacked at long range, and I used average damage. Calculation for every possible damage combination from all monsters would take forever. TaN is extremely useful against melee damage, as melee damage is typically lower than ranged damage and is also in the optimal range for TaN, because its just high enough that Red Armor+TaN 3 would reduce most melee damage to 1 hp, although there is still wastage against Former Humans, Lost Souls and Arachs, as their melee damage is tiny and will be 1 hp with Red Armor regardless.
Regardless of the assumptions, there are certainties:
Ironman is the superior trait against JC, CD, AVile, Baron, HK. No matter what armor you're wearing.
These are some of the more dangerous enemies. Mancubus is probably in this list too.
TaN is "probably" the superior trait against Cacodemon, Imp and usually Shotgun Former Human.
There enemies are less dangerous than the above.
Now, you might be wondering why I haven't said TaN is always better against Arachs, Former Commandos and Former Captains. These are all high damage enemies and worth a mention. The reason is because, if you're wearing Red Armor, you will notice barely any difference at all, as most of the damage they deal to you will be reduced to 1 hp by the Red Armor, and (I'm not certain, but quite sure) TaN doesn't reduce damage of 1 hp to 0 hp.
Of course, Red Armor doesn't last forever, and this is where TaN kicks in and could save your life. I just feel that Ironman is overall more useful as it doesn't rely as much on which enemies you face and what armor you wear, and its condition. Besides in optimal conditions, when one Red Armour gets damaged, you switch to an undamaged one.
In the best case scenarios, TaN is as much as a 33-100% improvement on your ability to withstand damage. We are mainly talking Arachs, Cacodemons, Imps and Former Humans types with Blue Armor/damaged Red armor. So TaN is good if Arachnotron is your worst nemesis. There is a lot of potentional for Arachnotron caves in the later stages, and you will definitely encounter more Arachs than Archviles, so its a valid argument for TaN. Personal preference I think.
Heh I'm almost convincing myself to take TaN, but I think I'll stick with Ironman. If you have a melee fighter, TaN is considerably more worthwhile IMO, but melee fighters typically take both Ironman and TaN anyway, and when you're berserking you can kill everything in 1 hit (in fact the chainsaw is 1-2 hit kill weapon anyway), however you could still get surrounded or rush an enemy and have him hit first, so you will see the melee use of TaN more.
