User blog comment:Unmei Suiheisen/Are Elemental/Damage boosts stackable?/@comment-26447088-20170214030320/@comment-26447088-20170215012839

Tubal Cain would be the better of the two overall (she's also the newer one of the two), however you wouldn't want to use Tubal Cain in your quest deck because of her lack of Multi or Rush skills, Pound typing (Pounds generally do the least damage compared to Slices and Flurries), and lower ATK stat. Personally, I'd rather have her as my Pound Assist from my Union mates.

Siren on the other hand, would be good as a quest unit due to her Flurry typing (generally do more damage), quite high ATK stat, and Multi with 35% rate during CTA events. However, her Passive is kind of meh (except for maybe in Fortress Frenzy where heals and burst gauge boosts are useful).

Both are quite cheap to build, so if you don't like one you can always try to sell them to help fund your purchase of the other. In your scenario, I would suggest building Siren, as Tubal Cain is just as useful as a Union Assist as a base as she is as a +2 (because you won't be relying on her to deal damage as an Assist).