I respect the humble, honest answer
Indeed. I'm late to the party here but as Vlad is a rather modest person, let me post a follow up for him
Mechanics:
Battle Brothers: simple surround bonuses (no facing considerations)
Warbanners: has more detailed positioning mechanics via flanking
Battle Brothers: blanket Night penalties
Warbanners: more involved light/dark penalties (different for each tile) - requiring good positioning for optimal bonuses
Battle Brothers: punishing Fatigue system that very strongly encourages choosing Fatigue on levelups
Warbanners: more Stamina is useful but not so vital that you feel forced to devote lots of points to it
Randomness:
Battle Brothers: can place you in randomly unfair terrain (all enemy units start on elevation and you don't), so battles can turn out quite differently
Warbanners: all battles are scripted and everyone gets the same deal. Fair is fair
Battle Brothers: hired men can be Asthmatic Dastards with stats well below par (even for expensive hires) and hiring good men is an almost entirely random affair aka lottery
Warbanners: hired men have standard stats, standard costs, so you always know what you're getting or not getting
Battle Brothers: damage is more variable, weapons have a damage range, critical hits are a feature aka "those enemy arbalesters all rolled below 5, hit my men in round 1 before I could move and killed them outright"
Warbanners: damage depends on Str vs Armor and is static for a given pairing, critical hits are not a feature aka "enemy marksmen can't 'lucky' crit you". In other words, if you fail, you mostly have only yourself to blame :p
General features:
Battle Brothers: men require upkeep and care: need to be paid gold each day, fed food each day, need medicine for injuries, etc
Warbanners: men are tough independent guys who can take care of their own meals and are loyal to the party
Battle Brothers: Reputation is an underused feature of the game
Warbanners: Karma affects certain potential hirelings and how the game plays out
Battle Brothers: no horses, no women, no sieges, no magic (player), no healing... and no legs :p
Warbanners: has horses, has women, has sieges, has magic, has healing... and yes has legs xD
Oh and lastly:
Battle Brothers: Diminishing returns on certain stats, other hidden mechanics/rolls and no actual manual till this day.
Warbanners: Extremely clear in-game manual with detailed but not confusing mechanics. Hardly needed to ask the dev to clarify anything.
And before you say I seem to be against Battle Brothers. I am not. It is a great game and people should play it too. If I were against it, I would not be helping to update the wiki for it.
However, Warbanners remains an underrated gem which should get more attention.