In my opinion they look better on the 3DS XL than on the original 3DS and smidge better than DS. Basically it's the same as if you were playing on a DSi XL but with a bit of smoothing added to try and make the 3D DS games not be super jaggy.
Overall 3DS XL would win my vote in best method of playing DS games. :)
http://www.youtube.com/watch?v=-fx4HYOunLc&feature=plcp 3DS FC - 3007-8283-2971