Many people believe the Edge to be a clone of the CycloDS.
The Edge is a good flash cart, but I prefer the Acekard 2 over the Edge. It has a much better menu system, and it's cheaper. There are no clone Acekard 2's so you're pretty much guaranteed of getting a real one from dealextreme.com where they're only about $5 more expensive than the DSTT, yet have one of the nicest menu systems of any flash cart at any price.
The only advantage I can think of that the Edge has over the Acekard 2 is that it's a little sturdier. If you're prone to stepping on your flash cart or throwing it across the room, you might consider the Edge. Otherwise, get an Acekard 2.
As to what makes a good cart: Well, almost every flash cart can sort games alphabetically and let you organize your games into folders as a bare minimum. Unfortunately, the DSTT does neither, and its menu is laggy when you have a lot of games. The Acekard 2 and Edge can also both load GBA games directly from the menu (if you have a slot2 GBA expansion cart like the EZFlash 2-in-1). The Acekard 2 also has a very responsive and pretty menu system, and lots of additional features such as multiple game save slots, the ability to copy/cut/paste/delete files, etc.
-Bri