Graphical tilesets and text graphics tend to follow completely different design philosophies.
Text graphics are representative. They represent concepts the same way the written word does. Just as we see the word "chair" and think of a chair, the same applies when seeing the glyph for a chair in DF. They represent the object in a completely abstract manner (often they sort of represent what it looks like, but only in a very abstract manner that makes it easier to remember, sort of like certain kanji characters in Japanese). No particular style is imposed, for the most part, and what the thing really looks like is up to the imagination of the player and other in-game (or out-of-game) textual descriptions and so forth.
Sprite graphics are descriptive. A sprite (or any sufficiently detailed tile) not only represents an object, but also has a particular aesthetic style to it, and tells you what it looks like. A tile for a dwarf has to decide what a dwarf (roughly) looks like, and what sort of style to depict him in. Are walls solid or do they look like brick-and-mortar? Do plump helmets have that sort of cartoony Zelda look to them, or do they just look like simplistic fungus? Are things in general cartoonish looking or ~GRIMDARK~ or quasi-realistic or what? There's an artistic style imposed on it that affects the atmosphere one way or another, and decisions are made for you what certain things look like.
So really, the two have to be judged in terms of not only their relative merits, but their relative purpose. Obviously, these aren't completely solid rules, as you can have simplistic graphical tiles that are still mostly for the purpose of only representing things rather than describing them, but text is obviously still more purely representative (as defined by me above). There's room for both, because they don't even work for the same reasons. To me, using a text tileset is more like reading a book or looking at something similarly abstract; I'm told what's there, and it's up to my mind to decide what it all looks like. Personally, I like that. I guess some might not.