You can make a rough evaluation in this way, that's not very serious, justa very rough idea:
Fight bonus from items =
(Leadership increase / Current Leadership) * (100 + troop talent importance bonus)
+ Attack increase * 3
+ Defense increase * (1 + defense importance in play style)
+ Int increase * Importance of magic in play style
+ Morale bonus * Number of troops affected * (Level average of troop affected/2)
With:
- Troop talent importance bonus = 20 if some troop talent linked to stack size are important to you / 0 if not (that depends a lot of troops you plan use, not many have such talent)
- Defense importance in play style = 1 if defense against enemy hits is important for your playing style / 0 if not (in general it's not for most playing style because you try avoid get hit and only by reduced enemy stacks).
- Importance of magic in play style = 0 to 2 depending of how important is magic. Only multiple of 7 and 15 are important so it's difficult to evaluate, this is an evaluation even rougher than other.
- The morale formula need to be reevaluate, but at this time I'm bored to check.
Then you get a number and can compare for two sets of items. But then you need look at special bonuses. For example:
- Poison dagger and whip brings huge damage bonus to low level troops but useless if not. Also poison isn't so efficient agaisnt undead and fire isn't efficient against demon and some dragons.
- The druid staff can be nice if you have many animals in your troops.