Elemental and Shadow are probably the most popular damage type defenses, both for the clockworks generally and FSC specifically. Just in case you've not looked into it, Chaos is highly prized for general use, except status-heavy places like FSC, where it's more for players very familiar with the level. If choosing between the Shadowsun and Nameless stuff based on DMG vs. ASI rather than based on the Shadow vs. Elemental, Bopp has a good point there but you'll probably be wanting to get secondary and maybe even tertiary armor sets before you start rolling for big desired UVs and _maybe_ even before bothering with Trinkets. It might also be sensitive to preference for weapons and particular ways of using them.

As Bopp mentioned, the Volcanic Plate is more desired for FSC while the Grey Owlite is more general-purpose, though they're similar enough that one can usually substitute for the other; the functional overlap is enough that if you get a second T3 shield after one of those Elemental ones, a first Shadow shield probably has more marginal value than the other Elemental one. Besides FSC vs. Shock, the other likely deciding factor between those Elemental shields is whether you find melee or ranged monster attack more troublesome: melee attacks tend to be Normal-only, while ranged attacks tend to deal the abNormal types. It's not an absolute rule, just a notable general tendency.

For what little it's worth, if I could tell my past self which gear to use for the final Wall of Heroes hurdle, I'd recommend something that probably sounds like blasphemy to most peops on the forum... like Shadowsun/Nameless hybrid, Volcanic Plate Shield, and Valiance, but having Volt or Prisma Driver, Strike Needle and Virulent Catalyser already at 4-star by then since they're my most important weapons after Blaster, so I'd almost always be using at least one of them at any time.