As an owner of both the Acekard2i and the DSTWO, I can safely say that I will never touch my Acekard again unless there is the unlikely situation that a game is not supported by the DSTWO but is supported by the AK2i.
While the Acekard is a phenomenal cart user-friendly cart for those looking for something that plays DS games, it really lacks in features in comparison to the DSTWO. Basically the only feature that it has over the DSTWO is Wi-Fi loader/cheat updating, which I have never used and will probably never find the need to (AKAIO hasn't updated its loaders for over 2 months now anyways). However, I do find it more user-friendly than the DSTWO, specifically the ability to list games by their 'Internal Names' and the ability to add a 'favourite' game shortcut to the root menu of your microSD.
The DSTWO is currently the commercial rom-playing flashcart with the most functionality and features. Even newer games like POP: The Forgotten Sands (which does not currently work with the Ak2i) work perfectly with this flash cart.
Save states are performed and loaded faster on this cart than any other, and it's surprising how useful the in-game menu can be when you want to check the time while playing a DS game. Although the GBA emulator for the DSTWO isn't perfect, it currently runs Fire Emblem and The Sacred Stones basically perfect with sound off, and it runs it with a slight frameskip with sound on. This trend generally applies to most GBA roms, and you can get basically perfect emulation with sound off.
The build quality of the DSTWO isn't the best though (the microSD slot is still miles better than the acekard). I've yet to have contact issues with my DSTWO like the ones I've had with my Acekard, but the DSTWO almost split into two when I took it out of its packaging since it's only held together by 1 screw. The cart is a bit thick.. and the only protection between the cart chips and the outside world is a small sticker. Some people have complained that it gets stuck in their DS slots, but I've had no problem with this happening and mine comes out just fine.
Lots of people have been complaining that a lot of the DSTWO's features are lacking... but I would rather underdeveloped features than non-existant features. The main features that people have been bitching about are e-book support, avi playback support, no current included snes emulator, and a lacking GBA emulator. I personally KNOW that all of these features will be fixed in the near future, specifically the first 3. GBA compatibility might be improved, but I doubt it will be able to play all games flawlessly without frameskip, ever.
Bottom-line: The DSTWO is a much better cart than the Acekard2i, although you could buy 2 Acekards for the price of 1 DSTWO.
This was written before AKAIO 1.7 though. With 1.7 out, I still believe that Acekard compatibility is just ON PAR with the DSTWO's. I also find that AKAIO 1.7's settings menu is a little buggy and it doesn't always save your settings now for some reason.
Bottom-bottom-line: The DSTWO has everything the Ak2i does, minus a few nitpicky things like listing roms by internal name, softresetting without using the touch screen, having rom shortcuts, or having a calendar in the main menu. The Ak2i doesn't have a lot of things that the DSTWO does like GBA emulation, an in-game menu (w/ clock), free cheat, real time guide, and exceptionally fast savestates. On top of that, SNES Emulation, PDF support, and DivX/XviD playback are still to come.
If you don't find any of the extra DSTWO features necessary, than the Acekard is for you. I just know that I'm probably never going to touch my Acekard again since I've gotten my DSTWO.
