Bug covers grass, dark, psychic
- There are psychic types in every generation, 4 legendaries at the moment
- Unfortunately, many grass types are grass/poison hybrids (poison resists bug) so the damage becomes neutral
- in terms of dark types, it hits neutral on houndoom and murkrow lol, steel/but both hit SE on Tyranitar, but covers umbreon
Steel covers rock, ice, fairy
- There are limited fairy types within the current generations. It will be more helpful in later generations when bigger fairy types come into play
- many ice types are ice/water hybrids (water resists steel) so the damage hits neutral
- is useful against rock types except the rock/water current and future hybrids (hits neutral)
I'd give a slight upper hand to bug CURRENTLY but in future generations they both have their merits
Edit: I love but type Pokémon <3~
Edit2: I think it with BP/IH is the best current dual steel moveset too. Another thing to keep in mind is TM usage: IH is guaranteed (fast) whereas you may need multiple charge TMs for XS