I don't know your games or budeget but I would go with the 3ds XL if you can wing it. The 3ds is the next system so it will have plenty of new games coming and it can play most of the old DSI libary too (all though the color is a bit muddy).
This is all depending on the games you like lets say you like the pokemon series and you want x and/or y then you want a 3ds but if you like older ds games get a dsi xl.
Also if you are going to get a 3ds get a 3ds xl the xl is amazingly better and has a huge screen