The R4 & M3 come out of the same factory. The only difference is in the firmware loaded on it.
I have a SuperCard MiniSD, and I have to dispute your claim there, Van, I believe the SuperCard is much better than the R4 or M3.
After flashing my firmware to run unsigned code (ie, homebrew), I can use my SuperCard for everything that R4/M3 can, and also
in my GBA.
My good mate here at Uni has an R4, and he only has bad things to say about Moonshell as the interface (personal preference, I guess), and only uses it to load DSOrganise
DSOrganise is an awesome little application - great for note taking, organising, browsing the net, chatting in IRC!
Before getting the Memory expansion, and trying to put DSLinux on the R4, make sure you have a look around to see it will actually work (every homebrew hardware is different!).
Having said all that, I may eventually get an R4, simply because it's cheap, and I want a backup homebrew cart.