I had a problem with this "choice" due to it being as contrived as it was. There was really no excuse why both could not be saved.
You had Arcann AND Senya with you. Given how powerful Arcann was, he could have easily lead the other two heroes you rescue along the way as part of the bonus mission (in my case Pierce and Kaliyo) to get Torian out. Hell, Arcann could have easily done it himself.
On top of that, you had all the other alliance forces you rescued just standing there doing nothing after you swoop in and kill all the enemies.
Bottom line, there were enough forces there to create enough of a distraction to extract Torian before Veylin even showed up.