The Sentenza/Obsidian Carbine have become practically obsolete ever since the introduction of the Winter Grave, no part in thanks to the Antigua's subpar damage potential and the Gremlin family's advantage in sustainability making for an innately flawed weapon design whose bonuses could hardly compensate for. The only good thing they have going for is that they can quite easily bypass a Mortafire's front shield with clever aiming, something that the Winter Grave will have a hard time getting through, but even all things considered the Winter Grave wins out for its ability to prevent Menders from doing anything, including resurrections and energy domes, with only a bit of investment in damage required.

Not only that, but the Winter Grave also wins out against the Sentenza/Obsidian Carbine when it comes to plowing through the likes of Giant Lichen Colonies, Polyps, and other Slimes worth talking about, as not only can it also prevent Polyps from shooting their barbs, but its overall stronger charge shot with a high chance of inflicting Freeze eases chaining charge shots while keeping a safe distance against Giant Lichen Colonies.

The Argent Peacemaker and newcomer Gilded Griffin have it much easier though, being the only ones that can quickly dispatch the overwhelmingly fatal hordes of Kats and Greavers, respectively, before they become a problem, all the while being decent against everything else that's weak to them.

That said, it could be easily argued that even they are obsolete thanks to the introduction of Arcana and Riftlocker, respectively, bringing more or less the same damage potential against Kats and Greavers while also being better at handling everything else. Furthermore, the Polaris/Wildfire have the unique niche amongst elemental handguns to prevent Gun Puppies and Howlitzers from shooting their bullets while also being able to effectively crowd control a horde into a corner of the map with its amazing knockback at range.

TL;DR, if you must absolutely pick an Antigua, then either pick a Silversix (which will later become an Argent Peacemaker) or a Raptor (which will later become a Gilded Griffin).

so basically Heater Shield or Aegis?

Neither. Stop upgrading them now. You should get Barbarous Thorn Shield and/or Swiftstrike Buckler for offense. If you want defense, you should get Grey Owlite Shield for shock and/or fire, or Dread Skelly Shield for freeze and/or poison.

Your Swiftstrike Buckler is good. Do not waste resources on your Defender/Aegis, even though you've upgraded it this far. Don't "throw good money after bad". Good luck.

Using a gun on gremlins is probably useless cause they dodge most of the time so does fiends (except for trojans which is more effective to use a needle for) and beasts so IMO they're both pretty useless but the argent peacemaker on the other hand is good against constructs and undead (especially undead) as they don't move a lot so unless you got an elemental gun you feel comfy with get an argent peacemaker (if you want a good gun for jelly get an obsidian carbine instead of a blackhawk).