There are currently 2 devices that are compatible with Animal Crossing: New Leaf, the 3DS and the 3DS XL.
If you you are looking for something cheaper (mostly), then the regular 3DS is the one for you. It is currently available in Blue, red, white, black, purple and the limited editions, Fire Emblem: Awakening and Legend of Zelda.
The 3DS is region locked so you CANNOT play software on a 3DS that is not from your region, I.E: Games from Europe cannot be played on a North American DS etc...
The 3DS XL however, has a 90% larger screen than the regular 3DS, as well as better sound. And is currently available in Blue + Black, Red + Black, Silver + Black, White, Pink + White as well as the limited edition Pikachu, Charizard and Animal Crossing.
Choose the one that suits you best!
(Other members can add anything if I missed anything!)