Personally, if I choose diplomacy, I always go for leadership.
Firstly, it allows you to recruit many more troops.
Secondly, it prevents morale from dropping when you have a diverse army.
And where did you get the idea that "with leadership, you recruit many more troops"? I think this isn't just a misconception, but complete NONSENSE! You might as well say that if you have creatures of the same type in your army as the neutral army on the map, it will give you an even greater chance of recruiting them – ABSURD!!!
Added after 3 minutes
I often choose diplomacy. I first realized what it was when I played on a strange map with the goal of obtaining the Sphere of Inhibition, and now I always choose it, especially if the commander is a Succubus. But there's a problem with artifacts – in the last Heroes game, the residents build everything for free as a reward for the Diplomat medal, so it's hard to collect a full set.:smile35:
But why even collect this "set"? Diplomacy artifacts don't increase the chance of monsters joining; they only make it cheaper to bribe them. ;)
P.S. By the way, why a Succubus?