I would go for the 3DS because Nintendo is done making games for the DS. And in the upcomming months there are going to be great new games like Animal Crossing New Leaf and Pokemon X and Y. Also the 3DS can play all DS games and there is also a feature called the Virtual Console where you can buy games from older Nintendo consoles and systems.
Get the 3DS XL. I wouldn't suggest getting the normal 3DS, it kind of hurt my eyes to look at the small screen. I've played on my friends XL a lot, and it's a step up for sure. Games I would suggest getting Legend of Zelda: OoT 3D, Resident Evil Revelations, and Monster Hunter 3 Ultimate. Buy those three games, and you'll be glued to your handheld for weeks.
I would recommend the 3ds or 3ds XL. The 3ds is have many new games made for it. But it depends on what you after. GBA has a nostalgic feeling and has many classic games and can play gamboy color games. the 3ds and 3ds xl can play ds games and have 3d graphics (I don't mean its 3d screen I mean like every thing is not all in one layer there is depth) and it can play games in 3d ( now I mean the 3d screen).