Hello there, My decision is to upgrade fatebringer because even though it has a maximum 300 base damage, the perks especially firefly take down targets very quickly. Aiming for a headshot with fatebringer causes an additional substantial amount of damage to the enemies who beleaguered the exploding target. The firefly perk for fatebringer is especially useful in raids like Crota's End and Vault of Glass. I prefer upgrading fatebringer first and then moving on to the Word of Crota. Hope this information helps