There's no concrete answer anybody can give.
The DSTwo is the current best solution, offering the best possible emulation and video playback solutions. For some that may be inadequate, but for others it may be great.
Although the CycloDS iEvo is coming, we don't fully know what it will be capable of, how will the DSi mode work? What advantages will it really have?
This is further compounded by the possibility that there will be a public DSi exploit very soon, which may or may not require additional hardware (see attached link)
In any case, to your question I can only pose another question, what matters most about your flashcart?
Are you most interested in being able to play DS roms, or are you really interested in the extra features?
If it's just DS games, I say opt for the DSTwo, since it has the best compatibility and is the most future proof flashcart available.
If you are interested more in DSi features, homebrew potential and more, you may want to see what the CycloDS iEvo or the DSi Exploit offer.
http://gbatemp.net/t271272-dsi-channels-exploit-announced