Mostly the fact that whenever I train either of them, they end up with crazy amazing stats without the need of stat boosters. For example, Jill in my first PoR playthrough got Str, Def, Skl, and Spd at maximum, while HP almost reached the cap. Sigrun was even better than Marcia when I trained her in RD, there were slight differences in stats, yet Sigrun had the upperhand. Mostly that. Well, and both are cute too (Jill more).
Favorite music genre?