The acekard is an excellent flash card solution.
However, after today's release of DSi firmware 1.41 there is a possibility that this card (or any, it is unknown yet at this stage how many of the cards are affected by this update) will have been blocked. Of course one can avoid updating to 1.41 but this provides other issues in itself.
Also another potential problem with the acekard is that in order to update the internal software of the cart, another DS on a lower firmware is needed.
My pick of the carts would personally be the M3i Zero.
It is completely updateable via usb, supports real time save and the list goes on. You don't really need me to list all the features do you?
This is my personally used cart and I'd say it is the best solution on the market, because it is upgradeable and also suits ANY so far released DS (DS, DS lite, DSi or DSi XL).
Hope this helps.