Dark Sould might have "better" combat, if you prefer it, because it focusses more on it. But as far as I understand the term "gameplay", it doesn't just include combat, at least not in The Witcher. We have many other mechanics, that are at least as important. Of course crafting and alchemy might be in DS as well (never played it, so it is just a guess), but thinking about choice and consequences is a gameplay mechanic as well.
Ok, so if I just focus on combat: As the question was asked "Would The Witcher gain from a combat like in DS"? Maybe, if you are someone that enjoyed DS and also enjoy The Witcher. To me, it would probably annoy me more, than it would add. I have the deepest respect for DS players, cause the combat would be too challenging for me. But it is just not what I am looking for. So for me DS combat in The Witcher game wouldn't be better, I would say, maybe even worse. I could probably tolerate it, if there was an easy difficulty, but let's not get anywhere near the "Should DS have difficulty levels" or "Should DS be more accessible to casual gamers"-discussion. I don't consider myself a casual gamer, but I am not looking for a combat challenge, when I play The Witcher, but rather for a good immersive, complicated and difficult story.
So, I think, we all agree, that there can be some improvement on TW2 combat. (And they seem to be on the right track for that, if you believe the interviews.) But would DS combat be better for TW3 or would TW3 gain from it? For some - sure, for others - maybe, for me or for each and everyone - probably not.
So in the end, it is a design decision for them. Do they want more difficult combat? Yes or No? I think, they rather want to keep the difficulty level and just improve on it, from a handling standpoint (interruptable animations, evasions...).
And as others said: It is all a matter of taste, which combat is "better" (meaning, which one I prefer more). Better can be a more valid attribute, if they improve the combat, like they plan, rather than changing it to DS, which would make it completely different. And for different combat it is a matter of pinions and tastes, which one is better, but responsiveness, ease of handling are things, that can be more easily compared.