You summarization of The Witcher is almost on the spot, except for the stuff about linearity. It's a beef of mine that non-linearity and roleplaying are tied together. But nevermind that.
But DA having good to very good writing? Seriously? Now it's my time to be weak, I admit that. I don't have the inclination to get into a lengthy discussion about the writing, 'cause that will take ALOT of time. Suffice to say, I disagree strongly with you. I will give some examples, but since I don't go into a lengthy analysis, feel free to ignore me:
1) Lelianas dialogue consists mostly of telling stories. These stories break several of the golden rules in writing; stuff even newly hatched writers know not to. For example, all her stories have no foundation; no reason they should interest us. They are simply "I went there, this happened, and I did this." There is no meaning in them. In fact, it's so bad, 90% of her stories don't even matter to the situation you're in, or say something about her. Overall, Leliana is the worst example, but...
2) The writing is extremely inconsistent. One time, a is said, and the next, b is done. DA's characters are mostly flat (Arl Howe being the prime example), so it hinges its quality on the nonflat ones. And every single one of the nonflat characters are inconsistent as hell. Loghain, Duncan, and so on. The inconsistensy is also there in the rest of the writing. For example, Duncan comments he can't act solely on a feeling in regards to the archdemon, but Alistair and other Gray Wardens can tell you it really isn't a feeling, he knows for sure.
3) It's packed with clichés, as I've already argued.
4) It has trouble with incentives. Real trouble. It can't figure out what it wants to focus on. Are you after the blight? Is it Loghain? The school example of this is Baldur's Gate 2: it lets you choose if you're after Imoen or Irenicus, and then fucks up the focus whenever it claims you're only after one of them. DA is even worse, since it won't even let you choose. This again leads back to the inconsitency.
5) It's plain bland. Almost all party dialogue has traces of "Leliana-syndrome", in which the characters will blather on about stuff that has no real meaning or relation to the story. When there are exceptions, for example Wynne's teachings, they are so stupid they make you want to bang your head against the wall. The only points in conversation with characters are as follows: "God is good," "Love will save us," "Serving your own ends is smartest," "There will come a time when you'll have to choose" and "X is your worst enemy."
DISCLAIMER: I haven't spoken much to Sten, Oghren or Morrigan yet, so I can't tell if they're different.
All in all, the writing in DA is BAD, all in CAPS. The setting certainly beats Baldur's Gate, as well as some parts of the writing and story (Landsmeet, for example). But other parts are even worse than Baldur's Gate, boring you to tears with filler-text.
In conclusion: Writing is a core part of an RPG. It's the most important thing, in my mind, since it determines what you react to, how you're able to react, and, indeed: What role you play.
A game with writing as abysmal as DA's sometimes is, will not make the top10 list in my book.
Again: The game is good. But it's not better than Bloodlines or The Witcher by far.