I have a G6 lite (w/ Passcard 3) and love it. The software can be a little flaky but its better then what I used for my GBA 😉 The M3 and G6 is made by the same company. It really comes down to if you only want to play games, or if you want to use your DS for music/movies. If its for games only get the G6, if you want to watch movies get the M3 so you can have different micro cards for music/movies.
Some people have problems w/ corrupted G6 carts because you have to stop the device (ie "Safely remove Hardware") in windows, but I suspect they miss that step.