Pixie, in my opinion better overall. It’s a safer option since it has a range of 4.6 tiles which outclasses all of the other swords in terms of range which is important to the Paladin since it is the squishiest of all the Melees. It also deals a high amount of damage with the Paladin’s buff until around 63+ Defense enemies (If all 4 shots hit) where Acclaim out-damages it. Even if you miss some shots, you’ll still do a considerable amount of damage. It also is readily available since it is a tradable item.
Yes, the Dblade deals a lot more damage than Pixie but unless the boss is stunned, Dblade is simply too risky on bosses than can instantly kill you you such most Events and End-game dungeon bosses. The range and spread of the Dblade makes it really bad at bosses like Daichi and all Shatters bosses since they shoot many high damaging shots in a short amount of time and are unstunable preventing any sort of damage being dealt with Dblade.
Oreo, solves basically most of the problems of Dblade for around 2 seconds since it’s Invulnerability effect makes you able to go hug any boss. It still is risky since if you don’t time it right you can still die. Oreo, however is, as you said, very rare since it’s an event white and isn’t commonly found.
Or you could just bring just use both of them as switch-outs.
TL;DR: Pixie is more useful for it’s longer range and tighter spread while dealing a still high amount of damage. It has 4 shots which allows can be useful if you have aim that’s slightly behind. Dblade is too risky since it has a low range and it’s spread requires you to go really deep. Oreo solves this problem by giving you Invulnerability but for a short amount time.