DS Lite. I only have access to my DSi sometimes, and on my 3DS, the Circle Pad doesn't work (It recognizes it as going so far to the left I can't even negate it by turning to the far right, even if it doesn't physically move left.)
Right now I have a DSi and 3DS so I'll be switching between those two systems. I would like to get either the DSi XL or the 3DS XL before BW2, mostly because of the nice screen size. It makes it easier on my eyes to look at the bigger screens and I like the feeling of the larger system in my hands.