The AceKard2.1 is only compatible with the DSLite and the DSPhat. So if you plan on buying a DSi anytime soon, don't get it.
The Acekard2i, on the other hand, is compatible with all Nintendo DS system's. This includes the new DSi XL, from what I know.
Oh, and if want to buy a AceKard, DealExtreme is probably the cheapest on the web, selling at $12.97 + free shipping. It might take a while, though, but it's generally a cheaper site.