This is probably unique to me, but Leliana taps straight into my own personal vulnerability: she sings beautifully. That decides the issue between her and Morrigan completely from my personal POV. My characters
may disagree (Or some of them do) but it's really hard for me to get past my own weakness on that one.
I tend to characterise Morrigan as 'wild fling chick' and Leliana as 'marriage chick' though that's definitely an over-simplification in both cases. It's the classic bad girl vrs good girl, but Bioware have given it a really neat twist in that Morrigan is the bad girl you can kind of win over and make into a good girl (Or at least a less-bad girl!
) because she's only a bad girl as she knows no other way, whilst Leliana is the bad girl turned good girl you can guide back into being a bad girl again if you want to. So there's actually an awful lot of thought gone into both characters, and neither is as transparent as they first appear. A thoroughly exceptional job all round, in fact.