So to start this off, one of the biggest factors is the price. 2DS costs $129.99, which is smaller than the $169.99 price of the standard-sized 3DS.
Score- 2DS(1) 3DS(0)
Next, the 2D capability. We all know the 2DS doesn't have 3D, which is why it is called the "2D"S. However, the 3DS 3D, in my opinion, isn't as great as I though it'd be. Yes, it makes it look cooler, but I can't look at it for too long without receiving a headache from it. Also, in 3D mode, the battery dies quite quickly. But, of course, you can leave the 3D off and just stick with the 2D. So here, just for the fact that it has 3D, the 3DS scores a point.
Score-2DS(1) 3DS(1)
Next is the design. The 3DS and 2DS have two completely different designs. To start off with, the 3DS is closeable, unlike the 2DS, which makes it more portable. The 3DS has the start and select button by the home button, which is a new layout for the DS brand. However, the 2DS features a start and select button underneath the ABXY buttons, which is the same as the other Nintendo DSs. I think in this case the old style is better, as it feels more natural (if you have owned a previous DS). Lastly, the look of it. The 3DS looks like a normal DS system. It is closeable (as mentioned before) and has the nice rectangle look to it. The 2DS, however, has a completely new look. Not only is it not the rectangle anymore (more of a square), it's not even flat, as it starts small near the bottom screen and gets fatter near the top screen. This is useful, however, because if you set it down, you can see the screen more easily, however, you cannot bend the screen to your preference like in the 3DS. The 3DS won 2-1 in the category, so the final score is:
Total: 2DS(1) 3DS(2)
In my personal opinion the 3DS is superior for its look, design, and 3D capabilities, which more than make up for the higher price tag