There's basically no reason to kill off Cain or Abel. I heard that killing off Jegan plus Gordin was good for some reason (which I would later find out is Norne), so I did that. Apparently the canon choice is Frey, but he's such a good unit that I don't really want to kill him off. Jegan isn't really needed on Normal Mode, too. Really, the game makes it pretty obvious that you should choose either Gordin or Jegan. Heck, they basically give you Gordin just to sacrifice. I hope this kind of choice comes back in a future game, in all honesty, where the game doesn't reward you for unit death. Ugh.