I prefer ASCII. Its also what I learned with. And I originally learned ASCII because I wanted to play vanilla. I will admit, originally I chose ASCII partly because I wanted the "pure" experience. I don't still think that way, I see points to both. Graphic sets can be pretty, although I see many with horrid color schemes, Obsidian's looks fairly nice. However, I might enjoy seeing barrels and hallways, the dwarves in all tilesets pain me on a personal level. Yes I do use 'v' to distinguish goblins from geese. Yes, 'g''s and '&''s don't look to menacing. But I always use 'v' and read the description, and when I do that the characters then come to life.
And thats what I really love about ASCII, its ambiguity. Its like the difference between reading a book and watching a movie. And as much as i love watching fight scenes with plenty of good CG, somehow the ambiguity in books to picture it however makes it better sometimes. And that feeling is strongest in DF, because I don't know what a pig tail looks like, nor how grand the dining room is. I rely on the dwarves to tell me something is a masterwork, and my imagination's inability to picture it has a subtle beautify all of its own.
So yes, originally I was biased. Now I appreciate some things about each. And lets be honest, stockpiles always look like crap, although I like the beauty of my differently colored rock pots in ASCII. But, in my opinion, nothing captures the beauty of ambiguity quite like ASCII. But then again, fights like this existed long before DF in the books vs movie wars, and will continue to be raged long into the future.