The main difference was their different value system. However, Morrigan's core tenet is to be independant, self-reliant and critical of what society has to say, whereas Alistair tends to believe in what is taught to him without much critical thought. So in that respect, Morrigan comes across as smarter.
Hmm, well then if Dorian is a romance he should fit this bill just from the little we know about him. He stepped away from his master because he didn't agree with what he was doing (if I remembering correctly, but I am on a serious sugar buzz from too much birthday cake) so that only would fit the independent, self reliant, and critical of what society has to say. But then that means Alistair fits too once he is hardened because he stops doing what he is told and does what he wants/ thinks is right. I have been very pleased with all the male romances in the past and I doubt I will be disappointed now no matter who is available. Every Bioware LI I have ever had, has been layered to me, deep and smart in their own ways. I could be a real sucker too since I would put Fenris right alongside Morrigan in terms of that hard exterior forged in the fires of life. But some people just don't like him and I have never really understood the depth of their dislike. :S I can't think of any companion, possible LI, that I have hated to the core. Once I fully took the time to talk to them, I could always see their side of things and respect it. That again, to point to my earlier comments about other things, is why I think the mystery of knowing who exactly the romances are is good. It forces people to actually talk to all their companions. So much context can be missed if you only want to chat with the one you want to love.
Both Miranda and Morrigan had a vulnerable side, if you talked to them enough I remember. So I guess back to the first bit, just because Dorian looks like he may as I said "be a lost kitten in the rain in need of pets and milk", doesn't mean he can't be an independent thinker willing to criticize you if he disagrees with something you do while remaining slightly distant until you really get to know him. I could actually say the same about Miranda when she talks about her sister or her self esteem about her genes or even Morrigan when she talks about the mirror. In those moments, when that wall is down... they are like lost kittens in need of some positive affection, romantic or just kind.