Urza has already answered in another forum that he believes the Acekard 2's menu makes it superior to the Edge.
Here is a comparison I posted to another forum comparing the Edge, Acekard 2, and DSTT.
SPEED: The DSTT might have a slight speed advantage over the Edge and Acekard 2. I don't personally think waiting an extra second or two for a game to load is particularly relevant, but you might.
FIRMWARE/MENU: The Acekard 2 has an awesome menu (essentially the same as the one they use for the more expensive Acekard RPG) that is nicer than that of the Edge or DSTT. DSTT's original menu (TTMenu) doesn't sort the files (YSMenu does, but it currently has its own shortcomings -- no download play, limited touchscreen support). Note that you can put YSMenu and the original TTMenu on the same card and switch back and forth between the two if need-be. The Edge's menu system is designed to look like the menu on the more expensive CycloDS, although the two carts are not made by the same company. The Edge's menu system doesn't use the top screen for anything (it shows a static image there).
QUALITY: The Acekard 2 is less sturdy than the other two. Not necessarily an issue if you're careful with it. A small number of DSTT's have had some hardware problems (some units don't make proper contact with the DSTT). If you want a DSTT, I would recommend asking the seller to pop it in a Nintendo DS just to make sure it's operational before sending it to you (many already do this -- it's to their advantage). Also, make sure you can exchange it if there is a problem.
CHEATS: The cheat system on the Acekard 2 isn't as convenient to use as the other two since you have to create a special cheat file from the menu before you can use a cheat for a particular game, and you have to re-create the cheat file for the game if you update the cheats for that game in the cheat database. The Edge has a proprietary cheat database format, so you can't update the cheat database yourself (you have to wait for the Edge team to come up with updates). If you use cheats, particularly for games that aren't already in the Edge's cheat database, this could be a problem.
MICROSD SLOT: DSTT and Acecard 2 have spring-loaded microSD slots, which means that you push it in until it clicks to pop out the microSD. With the Edge, you pry the microSD out with your fingernail. Spring-loaded is more convenient, but there have been issues with the R4's spring-loading where the spring would break, leaving the cart more or less useless. The same thing could presumably happen with the DSTT and/or Acekard 2, although it has been suggested that the R4's spring was particularly weak and prone to breaking.
There you have the main differences that I know of. Note that many of the problems listed could potentially be corrected with future firmware updates. The DSTT has had good support so far with fairly regular updates, and Yasu seems to update YSMenu fairly regularly. The DSTT is distributed by two different companies (one under the name TTDS and one under DSTT) which might increase reliability of updates (although it seems that only the DSTT team actually modifies the firmware currently). You can probably expect decent updates for the Acekard 2 since the Acekard RPG (made by the same company and sharing similar firmware) is updated regularly. The Edge is too new to know how good support will be for it.
Bottom line: all have advantages and disadvantages. Choose whichever seems best for you! Unlike the more expensive carts, none of these budget carts are a major investment (between $30 and $40) so if you choose wrong it's not the end of the world.
-Bri
