I would just wait for a flashcart that works in actual DSi mode as well as the standard DS mode. Until that happens, I consider all the current so-called DSi compatible flashcarts half-assed as they aren't technically "DSi" compatible, they just allow old regular DS ROMs to work in the DSi. Once actual DSi games get released, you'll be buying yet another flashcart for DSi compatibility, most likely.
My recommendation is to wait. If you have a regular DSphat/lite, just use that for flashcart stuff and use the DSi for official stuff for now.
But if you don't have a DSphat/lite, than I'd say you bought the wrong DS for the type of stuff you wish to do and you should wait for a FULLY compatible DSi flashcart to come out before buying one.
I won't even mention the fact that firmware updates for the DSi (something DSphat/lite owner's don't have to worry about) can possibly render any DSi compatible flashcart useless at any time, and that such an update will likely be forced on you before you can play the new DSi game online....oops, too late, I mentioned it.