Little math for fun.
If you get leadership every even level you get following amounts total:
Mage: 9600
Paladin: 12 000
Warrior: 14 400
I got no clue at all how much more you usually get from items, quests etc. But I am going to count that you get 5000 more than that as total. So 14 600, 17 000 and 19 400 leadership
Now 1 attack gives 3% more damage. Same 3% more damage you get by having 3% bigger stack. So 438, 510 or 582 leadership is what you need to do to gain equal amount of damage as 1 attack gives at end. Of course it is not nearly that simple. Bigger stack also gives some other benefits like more effective mind control for evil beholders, more hp so harder to kill whole stack etc. And of course bigger stack can use more money unless it's superior damage ends fights with less casualties.
But I would say that it is probably best to start taking leadership instead of attack at latest on level 10. Or maybe at 8.
I think attack is clearly better stat to have than defense since at least I try to be one hitting way more than one getting hit. And there is lot of slots to put defense items while attack is much harder to raise.
Int is really hard one. It raises direct spell damage about 10% for each point. But since I have not played past dwarves I got no clue how useful direct damage spells are at end and how much you instead use buff spells which gain more power bigger your stacks are.
Rage and mana are also very hard to estimate. You need certain amount to operate. One thing to remember is that you can get total 6 roasted rats by talking to dwarf miners. Those rats give +2 rage and +2 mana with cost of 50 leadership. So you can trade 300 leadership to 12 mana and 12 rage. Definetly good deal since even mage gets 320 leadership at level 8.
All this just to figure my own strategy for next game.
I think I try taking leadership at level 8 and every level after that when it is offered. Level 8 also gets me leadership before pirate islands and one very soon in there.