DSs are not expensive these days, especially compared to a tablet.
Run down of the different types:
DS - Original model, can play DS games and Game Boy Advance games. Region free
DS lite - Just a smaller version of the original model
DSi - Upgraded DS. Can play all DS games*. Can't play Gameboy Advance games. No longer region free (though it gets a bit complicated). Comes with 2 cameras (one facing you and one facing away). It's also possible to download "DsiWare" games from the internet. There are certain games that are "DSi enchanced", these can still be played on a regular DS/lite, but when played on the DSi have extra features (such as more secure internet options, or video chat). There's also a handful of games that can only be played on the DSi (about 5 if that).
DSi XL - Bigger DSi
3DS - Plays games in 3D. Can play 3DS games and all DSiware, DSi and DS* games. Also can download old nintendo games from the internet (from older consoles like the NES or the GameBoy Color). Not region free
3DS XL - Bigger 3DS
*Can't play DS games that require the GBA slot of the original DS/lite, such as guitar hero.