As the title say..:P.
Which line for antigua?
There is currently a thread, started by the game developers, proposing changes to dual-damage weapons including Argent and Sentenza. I would wait until that shakes out, before spending any resources on either line.
If you insist on upgrading, get Argent Peacemaker for undead and fiends (Zombies and Trojans) in Firestorm Citadel. The Sentenza is a little better now that beasts and gremlins aren't as agile as before, but it still can't do its own intended job.
That proposal was just an idea, so the Antiguas might not change (but I hope it does). Even if it does change, AP in my opinion is still better since the enemies it is effective against do not dodge gun shots while Sentenza would still make Gremlins dodge.
AP for FSC and Sentenza for LD. Your choice....
I'd recommend the elemental line (Argent Peacemaker), but after the latest patch, I'm not going to recommend it quite as strongly as I would have before.
AP hits well against undead and fiends, which is very useful for the enemies that appear in FSC.
Sentenza is now much more likely to land a majority of its hits on Wolvers and Gremlins after the patch, which makes it especially good for sniping menders.
I myself have an Argent Peacemaker that I use and love, but that was before the latest patch, in a time where dodging enemies made Antigua lines difficult to use (and thus made the Sentenza pretty much useless). I'm currently working on a new Antigua to make into a Sentenza to try it out on menders in arenas. My former choice was a Biohazard, which was a great way to herd menders backwards out of the middle with the slow projectiles, but that doesn't work so well anymore (again, due to the new patch).