The unsaved souls of the dead unborn are not tortured; they simply exist without communion with God, because they never had the chance to know Him, or to come into actual communion with Him through the provision of divine grace and the virtue of charity, which is necessary to salvation, via the sacrament of baptism. Hell is punishment, and heaven is reward. Limbo is neither; the dead unborn go there because they do not deserve punishment, but nor do they deserve reward.
Which is more merciful? The former. But which is more just? Neither - because as I have explained to you here, the aborted do not go to hell, and do not consciously suffer. Instead, they are held in limbo; a state of neutral suspension. The premise of your question is wrong.