Don't get it if you want to use DSLinux. Doesn't work with it.
Sorry for the gravedig, but I just now saw this. It doesn't work with the R4? The DLDI build when used with "dslinux.nds" and not "dslinuxm.nds" should work fine on the R4. I've heard reports of it sometimes only working with FAT32 cards at some point, but I think that issue got resolved in the latest builds.
Regardless, the R4 would be a good choice, as would the CycloDS Evolution. The Supercard DS has some cool advanced features you can't find on other carts (like the ability to consult a gameFAQ guide in the middle of playing), but it's not the most user-friendly device, so new users may be put off by its sharp learning curve.