First of all, you should probably never choose Sentenza over Argent Peacemaker (aka AP) when you want to upgrade Antigua unless PvP (with weapons you have, not bomberman) comes out. Second of all, even if you upgrade Antigua to AP, it is only going to be useful in FSC and other Undead and Fiend (Devilites excluded) Stratums because it deals split Piercing/Elemental, and since monsters have more resistance then weakness, Silversix is only effective at Undeads and Fiends (and Devilites dodge guns by the way).
Pulsar is good for FSC if you upgrade it to Polaris since Polaris deals Elemental damage while Supernova deals Normal damage. And Polaris is just excellent, I have Gigawatt Pulsar (the 4* of Polaris) right now and I love it, just note that the Pulsar guns deals knockback on the expanded bullets which is going to annoy your teammate because their attacks on the monsters will miss. Since Polaris does knockback, it is extremely useful for keeping enemies away at one of the FSC places where guns gets very handy (hopefully everybody knows which place I am talking about, the platform between two sets of spikes and where Zombies appear behind the spike).
For Vanaduke himself, you would want to use a Piercing gun which means AP, Blitz/Plague Needle, Callahan on him as Piercing damage is effective on Vanaduke. So Polaris for the levels before Vanaduke, but AP for Vanaduke. AP is good for the levels before Vanaduke too, but not as good as Polaris.
I definitely won't regret bringing Polaris into FSC instead of AP. And also, someone in your FSC team will definitely carry Silversix or AP so it is really no need to bring another one.