User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-82.178.240.134-20180620013832/@comment-10330822-20180620062746

First off, Balance Breakers by definition are "forbidden power" that wasn't meant to be used by the Sacred Gears created by God. Azazel explained they are "bugs" in the system that result in strange and unique power-ups.

Balances Breakers are broken down into three known categories by Azazel; Crest Side, Abyss Side, and Ex Side Balance Breakers.

Crest Side consists of normal, traditional balance breakers for Sacred Gears. It also consists of "Subspecies". Subspecies are basically distances from the regular Balance Breaker. Of course, they can exist for the normal Sacred Gear states as well. A good example would be Cao Cao's True Longinus or Kiba's Blade Blacksmith, who has a subspecies that differs from the regular form. Meanwhile, Sigfried had both a Subspecies Sacred Gear and Subspecies Balance Breaker. Despite this, its still considered Crest Side. Subspecies usually appear due to rare affinities and strong feelings of the user causing the Sacred Gear or Balance Breaker to develop differently than normal.

An Abyss Side is so much a different form as it is an improvement on a normal balance breaker or subspecies. For example, Tobio has the normal Balance Breaker for Canis Lykaon, but also because of his natural affinity and dedicated training, has an Abyss Side form as well. Abyss Side is basically a fusion of wielder and Sacred Gear.

Ex Side is basically that which doesn't fall in between either. That includes "irregular" Balance Breakers like Kiba's Sword of Betrayer. Azazel stated that Issei's Illegal Move Triaina and Cardinal Crimson all fall into this category as well. They are basically exceptions among exceptions. They are basically bizzare, rare Subspecies.

As for your question as to which is better, it depends on the Sacred Gear. Cao Cao subspecies Balance Breaker is extremely powerful because he has the highest-tier Longinus. However, normal Subspecies aren't necessarily anymore stronger than normal Crest Side Balance Breakers. Abyss Sides are logically more powerful than most subspecies because it is "a step beyond" normal balance breaker.

As for irregular, they are outliers, so its not really fair to compare them. However, like other Subspecies, it would really depend on the kind of Sacred Gear.

As for Sword Birth, its hard to say. There is still so much more that can be explored with Kiba's abilities, especially since he has two very similar Sacred Gears. As for surpassing True Longinus, that might be overreaching. But the chance it may get re-classed as a Longinus is always there.

Forbidden Balor View was re-classified as the Longinus, Aeon Balor, for example because of Gasper's rare situation. When Gasper's "balance breaker" emerged it caused an "irregularity" similar to Kiba's Sword of Betrayer where it is hard to exactly define what it is. Ultimately, it's existance boosted the Sacred Gear to Longinus-levels. Whether Sword Birth or Blade Blacksmith is capable of something similar remains to be seen.