The following post clarifies a lot of the mechanics:
https://www.reddit.com/r/totalwar/co...or_and_damage/
For example, the chance to hit is "40% + Attack - Defence", capping out at 10% and 90%. Armour reduces 75% of its value from non-AP damage, on average.
Knowing this, we have a maths basis for comparing +9 attack vs +15% weapon strength. In battle with another leader, WS should be better because 15% is more than 9%. However, against rank and file with low HP, extra WS may be wasted as overkill.
Similarly, 15% more hitpoints is a bigger boost to time before death than -9% chance to be hit (although Guomingdong makes an interesting point about stunlocking animations). Both seem better than 15 points of armor in a duel with another warrior leader, which reduces 6% of non-AP damage if, for example, Karl Franz on foot was fighting his mirror. More armour would also be a poor choice for a well armored lord fighting rank and file: Karl Franz would already be negating all the non-AP damage.
Having read this thread and reflecting more, I think I'd change my preferred build and go defense heavy, choosing 15% extra health from the first part of the tree and 9 melee defence from the second. (My earlier preference for WS was based on its pre-nerf 30% bonus). Your Lord fleeing or dying is a disaster, so I think that would normally trump offensive boosts. Note that for Karl Franz, at least, points in HP and melee defence are not competing within the same part of the skill tree.
There's enough good non-warrior traits that I would put the minimum in the yellow tree to unlock it's final talent, deadly onslaught (18% AP damage plus more charge bonus for 36 seconds). Perhaps one point in attack (+2% to hit) and one in weapon strength (+ 3% damage), but due to increasing returns to talents, a solitary point in any of them is not very exciting.