for sake of argument, lets math out str 8 board torp wave, against 4+/5+/6+ armor with both +1 and reroll.
With the +1 to hit
5.333 hits vs 4+ armor, 4 hits vs 5+ armor, 2.666 hits vs 6+ armor.
With reroll to hit.
6 hits vs 4+ armor, 4.4444 hits vs 5+ armor, 2.4444 hits vs 6+ armor
so reroll is generally better vs 4+ and 5+ armor, but +1 to hit is generally better vs 6+ armor.