Choice Specs is an interesting option for Zygarde to lure and take out physically defensive foes and Tapu Bulu. Choice Specs-boosted Sludge Wave has a chance to OHKO Assault Vest Tapu Bulu and 2HKOes Clefable and Assault Vest Tangrowth. However, this set's general lack of power and inability to effectively use Thousand Arrows makes Choice Specs hard to justify.
Choice Specs Zygarde isn't totally unviable, as it is able to lure in a few physically defensive Grass and Fairy type Pokemon such as Tapu Bulu and deal heavy damage with Sludge Wave, which Choice Band Zygarde cannot do.
However, other than that, special attacking isn't as good as physical because:
- Special attacking Zygarde misses out on Thousand Arrows, which sets it apart from other Ground types thanks to its ability to hit flying types such as Zapdos, Skarmory, and Celesteela. (3 Pokemon Pokémon that it could beat pretty easily before is now completely walled by it)
- Zygarde has a greater base physical attack stat (100 vs 81), therefore hits harder.
- Not only is Zygarde a good Choice Band user, but it also has many physical attack-boosting moves, such as Dragon Dance and Coil, meaning a physical Zygarde has more viable sets
So, while Choice Specs Zygarde is very unpredictable and can beat several threats that otherwise wall a physical set, it will do worse against many other Pokémon. Therefore, I would stick with a physical attacking Zygarde, unless your team requires a Choice Specs set for whatever reason. However, if you do decide to use a special Zygarde (which I suggest not doing), HP-Fire is better than HP-Steel.