Annihilator. Gastra cant proc its OD2-3 on Barrage, whereas Annihilator can proc its x2.5 on Barrage (15+k barrage). Also its a Crossbow/bolt vs Gun/Bullet, so gun has higher potention for higher damage, and easier to get 4-5 hit builds.
If you're gonna pick one of those, Annihilator for sure.
Way easier to make (1\5, 1\6 ish the price? + way less other stuff to do)
Better from a pure DD point of view, and it's an amazing low-hate tool in situations which that might be needed (Odin 2, and probably future fights)