3D image quality is debateable between the two, it's very close. Personally I find either card could have the slight advantage depending upon the individual application.
DVD image quality difference is slight, but noticeable in ATi's favor.
2D image quality varies dramatically between manufacturers of nVidia boards, so it's hard to say but ATi is IMHO very slightly better then the best of nVidia board manufacturers in 2D and considerably better then some of the lower tier manufacturers.