First I have to say, awesome topic way to go OP and great conversation by those that have followed.
Anyway, it's pretty hard to say which bloodline would have the larger following in the end. You have the legend that is Loghain which isn't far short of the legend of the Warden. I think the Couslands would win out in the end just due to their long history in the realm. I'm not going to make any qualifier like "assuming children", because the fact of the matter is both sides die away without someone producing children after the blight. The Couslands might have some distant relative from a marriage to a Bann or Arl at some point however, not so with the Mac Tir as they only have 1 member of the second generation, Anora.
So its either the Couslands or neither.
I love the Cousland Origin, probably my favorite.But it certainly made for a tough choice in my case. I started out really liking Alistair, funny guy. But in the end a bit of a whiner and certainly no leader. He admits so himself outside Lothering, and personally never thought otherwise. But he makes a good friend. On the other side, I despised Loghain, dirty stinking traitorous rat.
So if Alistair is no leader the Landsmeet rolls around and people seem to like Anora, so a Cousland joining her on the throne didn't just sound appealing but seemed right. But then it comes about that you can't marry her and get rid of Loghain, not only that you spare Loghain and Alistair flips his lid (rightfully so). So my playthrough... Exiled my friend, spared the traitor, and became Prince-Consort/Warden Commander/Hero of Fereldan... Oh and did I mention this was after falling in love with Morrigan, making the OGB, then disappearing from the Denerim in search of the lost mysterious love.
Lol you can't get a more complicated combination of events but that's what I love about it. I don't think there is any other RPG in the world that can give you that kind of complicated (if melodramatic) depth.
I have to say, after reading the books though... Loghain is the baddest mf'er out there. The ultimate warrior and general. However I think the Loghain from the books and from DAO are 2 different characters which is a shame because it would have made DAO so much more interesting if Loghain had that extra depth. But as it was, as hateable as he was Loghain was a far better villian them the vast majority in any other game.