If the ironcurse icon saves your steggie once in twenty or thirty battles it pays for itself and any crew saves are just bonusses. The iron curse icon is so useful and so cheap that you almost have to never face a war machine for it to be a waste of points.
You're limiting your options of gear, for an item that your mount already outclasses.
Ironcurse grants 6+ ward against only warmachine fire, and only to the bearer and his mount
Portent of Warding grants 5+ ward against anything not close-combat in an aura
My EotG spends most of its time in the burning alignment so the portent of warning only helps right at the begining of the game. If you want to have the portent of warding up after first player's second turn you are doing something wrong (or facing a gunline).
Maybe I don't wanna charge headlong into combat against some armies, and can be content to punish them with magic and shooting first. Also, Burning Alignment is so short-range you'll almost have to be in combat for it to be worthwhile.
The whole reason you're taking Ironcurse is to stop warmachines sniping the Steg. So, by definition, you'd want Portent up against the same kind of armies (Dwarves, Empire, Skaven). Hence my point, you have 5+ ward included in your price tag, buying a 6+ is pointless.
The ward on the priest only ever affects the priest, but the EotG loses so much of its power if he dies that the ward does far more for a priest on a steggie than it does given to any other character.
I don't see how. Priest has only two T2 wounds to give up before he dies. I'd rather give the 4+ ward to an Old-Blood or even a Scar-Vet TBH.
As I said before, if your opponent wants the Priest dead, all he has to do is charge you. Once in combat, he can choose to attack you directly, and if he has high Strength or a weapon that does something similiar, you're dead.
Priests are better for loading up arcane items on anyway. Slann will always have Cupped Hands, so your only other place to put stuff like dispel scrolls, Blood Statuette etc is on Priests.