I've done some theoretical research on the three primary FSC shields, and figured out what they're ideal for.
Crest of Almire:
- zombie swipe
- zombie lunge
- Trojan sword slam
- Trojan charge
- Trojan attack buff
Grey Owlite Shield:
- Red Rovers
- Rocket Puppies
- elemental grates
- shadow fire
- Vanaduke's mask bullets
- falling ceilings
Volcanic Plate Shield:
- Vanaduke's mace
- slag guards
- spiked wheels
All take reduced damage from zombie breath.
Which shield to take through FSC depends on your circumstances. If you're getting slammed by FSC's numerous enemies, then take a CoA. If you're lagging a lot and/or tend to run into things, then take the GOS. If you don't quit at the end of the Smouldering Steps and decide to go for the boss fight, you'll probably want to take the VPS. Note that the CoA resists moving targets, the GOS stationary ones, and the VPS boss attacks.
Any opinions?
Barbarous Thorn Shield for the sword damage bonus if you are a swordsman, swiftstrike if you are a gunner. If you are a bomber, well cry to OOO for not having any offensive bomber shield.
Once you are experienced enough, you simply don't need that much your shield. For a swordsman, the one or two hits the BTS can take is enough. A gunner should totally kite and not use his shield for anything else than shield cancel and shield bump.
On another note : Crest requires Almirian seals, which requires you to be farm FSC quite a bit. By the time you have enough tokens, you'll probably already have a good enough shield or even be able to do it with whatever shield you feel like.
If I had to take one of the 3 you said, it would be Owlite, because the mask bullets are a little troublesome, and because the VPS doesn't last much longer against anything than Owlite, although Owlite clearly withstand much more elemental. I tested a while ago, both will break one the second mace hit, and you'll have the status. The only difference is you are going to take less damage with VPS on that second hit (perhaps no damage at all ? I don't remember, you definitely will have the status).
But in the end, the offensive bonus of BTS is more valuable to me than a shield that last a little longer. Because I don't let my BTS breaks at all (and if it does, I deserve it, and it's not the end of the day).