400 dps to 500 dps bow is a massive, massive jump. This is a 20% base damage addition which is the damage pool from which you will scale all of the rest of your damage multiplicatively. Unless the extra projectile can exceed that damage increase, go for the base damage because that will scale all of your arrows instead of just adding one more. This is unless you have special interactions like chaining.
Yeah im unsure about how much more dmg that extra projectile would give me and if thats more helpful for bosses and map clear...