When comparing War Party and Well Trained, one has to also include non-numeric differences.

Well-trained Thralls hit like a truck. Their DPS may be higher than that of two war-partied-thralls, especially if you also use equipment and buffs to increase thrall damage.

War-partied thralls, however…
… stagger more,
… allow to use different utilities, e. g. a bearer thrall and an entertainer for farming runs, or a fighter and an entertainer to fight in corrupted areas and
… stack different debuffs and dots, e. g. one thralls with a poisioned and one with a bleed weapon.

Well-trained thralls…
… use up less space, i. e. block your path less often,
… need less equipment and
… are less in danger of being killed at low levels.

Personally, I prefer war party, as it allows to level two thralls at the same time. Also, the player character seems like less of a whimp when accompanied by two less powerful thralls instead of one super-human.