Those Military NIs are really confusing and Strategy Guide really fails to explain them in detail.
Now I'm considering Tribal Recruitment too since my manpower pool is huge and getting bigger force to the field each time seems as useful as getting there force with higher morale, force with higher discipline...
I checked and AI seems to be split between Professional Soldiers and Horse Lords. So they pick better discipline over morale from Martial Ethos.
So now: if I take Martial Ethos I have +0.25 morale and they have +0.10 discipline, if I take professional soldiers, we're even... If I take Tribal Recruitment, I have bigger armies in the field.
If I take Military Camps I will lose a battle but will be able to strike back with full morale quickly.
I think I'll go for Tribal Recruitment and see how it works...
Anyway, what do you guys pick?