AK2.1 and 2i are very similar, the 2.1 only works with the DS Phat/Lite.
The 2i works on the DSi and can be updated to work with future firmware updates.
Beyond that, they can both run AKAIO and have similar build quality. (2.1s had a 'bad batch' that has been addressed, the 2is seem to have a thinner shell and has some contact issues which is fixed by slipping a piece of paper under the PCB)
If you're going to buy the card, I would HIGHLY recommend
NOT buying from a site with 'R4' in their name.
Some places I would recommended are
ShopTemp (Affiliated with GBATemp) and
GameKool (I ordered mine from them, got it in 8 days).