So, I will start with what I most enjoyed of the two. Even though, the graphics and loading times let it down quite a lot, the original Nintendo DS version plays it better for me. The camera controls are much easier to operate as you use the directional buttons to move around within the play aera, buttons A,Y to rotate the camera and B, X to zoom in and out. This is a lot easier to adapt to that what the camera controls for the 3DS have to offer. On the 3DS version you do not use buttons to move the camera, in fact, you do not use buttons for anything, marking them useless. The 3DS version offers a control system where everything operated on the touch screen and I find this rather annoying. When I first brought the 3DS version I expected the camera controls to similar to that of the original DS version, my predictions were far from right. A thought that srung to mind however, is that this may just be OK and bearable on the 3DS XL as the screen will be a little bigger leaving more room for all the controls that have been tightly packed in. At least the plus side is that the graphics are much better to that of the original DS's especially when you play with the 3D feature on full. Admittedly, I was fairly impressed with that after looking at the pixel-ed preview images on the back of the box.
One thing, I should mention is Creat-a-Sim modes. The 3DS wins this purly for the graphics and extra facial design options whereas in the oringial DS version the graphics really do let it down and as the above suggests there are less options to get creative with.
My overall summary is that The Sims 3 for the original DS has better control making it more addictive for your Simming needs but, the 3DS version looks much prettier.