I say DSi compared to what anyone says mainly for these reasons:
1. Cameras - It may seem stupid and a waste of money for a camera, but the camera can be integrated into games, even still making them compatible with DS and DS lite. How? Take a new wario ware. It comes with 50+ minigames, but if you happen to load it in DSi, you get extra minigames which use the camera, say to pop bubbles with your hands rather than a stylus, or maybe unraveling a toilet roll by making a downward motion. Then you could do party mode, where it uses the back camera to motion sense several people, lots of fun at small parties ;-)
2. DSi Shop - This may seem a rip off from the wii, but this is quite good. Going on from what I said above, people could make DSi only games which mean doing something like eye toy maybe! You could also get stuff like applications to download, and use, so maybe even if you have no games, you can use them. Scenario time: You has your DS lite in a car, with a DS game in it. None of your bros/siss have a DS. You suddenly realise you are bored with/beaten the game, and turn it off. Now you have nothing to do for the rest of the journey except have no life in pictochat ;-). Now, new scenario: You have your DSi, and a DS game inside. Suddenly you feel you are/have bored with/beaten the game. Now you turn off, but you can still take a pic of the scenery, make a movie in moving notepad, then turn your sis into a gay freak XD. See? Now if you read that, you have as little life as the guy in scenario one XD.