The Nintendo 3DS is a completely new console and different than the DSi XL. The Nintendo 3DS is capable of producing 3D images on specific games. The 3DS has a virtual console for GameBoy games and a 3DS shop. The DSi XL is now the old system. The screeb is much karger than the 3DS. That is the only advantage the system had. The 3DS can play both 3DS and DS games while the XL can only play DS. If your stuck between the two purchase the 3DS.
The 3DS is a new system that has better graphics including 3D. It also has a 3D camera. You can play cartridges for the DS on the 3DS but they won't have 3D graphics. You cannot play cartridges for the 3DS on the dsi xl.