I have both Raspberry and Arduino.
What's is easier to use for this little project? I mean, what will need less parts or mods?
Thanks!
for the arduino you need a fifth wire for the HW mod (which shouldn't be a deciding factor if the 3DS isn't already modded as soldering it in while the 3DS is already open will take about 5 seconds).
the raspberry pi needs an sdcard, an usb stick and needs to be used either over ssh or with a hdmi monitor/tv and usb keyboard (for which you will need an usb hub too if you got a raspberry rev A with only one usb port).
so the pi is only favorable if the 3DS is already modded using the 4 wire method and you don't want to open it up again.