You want the 3DS.
I prefer the smaller size of the 3DS because I like to keep mine in pocket at all times for on-the-go play and streetpasses. The 3DSXL's increased size makes this more difficult, because that thing is goddamn huge. (And no, I don't wear skinny jeans, before someone says that.
I am nowhere near the appropriate weight or body type for that to be in my wardrobe.)
The 3DSXL screen is a bit too big. I feel like I have to play the system at arm's length to see the entire screen. This can become a bit uncomfortable and awkward, even moreso considering...
...the 3D effect is easier to keep in focus on the 3DS. Because the screen on the XL is bigger, I have a tendency to hold it further away from my face, which causes me the break the 3D effect. When I bring it in closer the screen overtakes my field of view, which is awkward and uncomfortable, so I try to hold it further away...this breaks the 3D effect again, and thus a cycle is born.
The screen on the 3DS was designed to be the size it is on the original 3DS hardware. The 3DSXL has a larger screen, but isn't higher resolution, resulting in big ol' chunky pixels. Frankly this is hideous in my opinion, and it makes beautiful games a disgusting mess on the larger screen.
I prefer the classier "Sleek and Glossy" appearance of the 3DS hardware versus the "Toy-Like" 3DSXL hardware. The 3DS feels like a professionally made piece of electronics whereas the 3DSXL definitely feels like a "My First 3DS" product.
Speakers on the 3DS are louder and clearer on the 3DS than the 3DSXL.
The battery is expandable on the 3DS. Now I personally don't have any issues with the 3DS' battery life like some do, but if I wanted to get an extended life battery for my 3DS, multiple solutions are available. The 3DSXL has no such option to my knowledge.
I just enjoy the color treatment on the original hardware better. The tri-layer design and the beautiful gradient-tone lid just look amazing.
The 3DSXL's all-around flat colors are just boring by comparison.