Table sugar (sucrose) converts to simpler sugars like dextrose in the presence of acids. It's used in wines alot, as the pH is typically lower than in beer.
I'm not certain how acidic the apfelwein is, so I can't tell you there. In winemaking from grapes, there is typically pH's in the neighborhood of 3.1, so they convert the sugar easily. Normal beer isn't that acidic, so I guess that's why the flavor is affected. But I think apples are pretty acidic.
As far as amounts, since dextrose (corn sugar) sold in homebrew shops has some water bound up in the molecules, you use more dextrose than sucrose. I saw a figure of about 8.9 % less sucrose than dextrose by weight, which I used in my last wine adjustment. Other places on the internet say the difference is 20-25%! Perhaps the first is fermentable difference, and the second is perceived sweetness? Perhaps someone can clarify this for us.
As to flavor differences, most in winemaking say you can't tell the difference, and sucrose is typically used there. I used dextrose because I had it, and my wine started a little high in pH (I later adjusted it).