i don't have a ds (or a R4) i'm going to buy both, and i've already investigated (it's really easy) i've found the R4 is pretty similar to the "M3 simply" both are the best you can find in market today, the R4 is slightly better since you can choose from different skins and you can actually remove the microSD memory without taking the whole thing out, that and a couple more details make it better than the "M3 simply".
You must know that every slot-1 flashcart ONLY allow you to play DS roms,
no GBA roms, however you can play nes and snes games with homebrew and DS roms, images, videos, music and freeware (homebrew). The Videos need to be converted to a special format with a software you can find in the R4 homepage.
i found 2 great reviews about these two devices but they're both in spanish, so i have just resumed it to you, and i'll post this videos of the R4 working.
Dead n' furious game and One Piece intro video on R4
playing a slot-2 game from the R4 menu
R4 Multimedia test
PICS
R4 Main Menu
M3 simply inserted in a DS Lite
M3 simply Main Menu
I hope this was useful for you ;)
Source (and credits to):
ElOtroLado.Net
http://www.elotrolado.net/showthread.php?s=&threadid=684663
http://www.elotrolado.net/showthread.php?s=&threadid=678145