I don't do a lot of video so I'm not speaking from extensive experience. The manual audio controls on the M and 6D make it easier to adjust levels to a particular microphone.
I've got/used a Rode Stereo VideoMic (non-Pro & Pro) on 550D/6D/M and been pleased with the results. Much better than the internal mic setup, particularly with IS lenses. I prefer the smaller form factor of the newer Pro versions - gave the non-Pro version to my kids.
I recently picked up a Shure VP83F Lenshopper and it works well in my initial tests. Hoping to record a kids Christmas event at church with it this week on the M while I shoot stills with my 6D. Wanted a directional mic in this case and like the option of off camera recording with this mic while the Rode is mounted on the camera.
I haven't gotten around to trying an old Sony lavalier mic setup I used eons ago with my HandyCam. A situation just hasn't presented itself yet.
Wind can still be your enemy outdoors, even with a deadcat wind muff. Here's the Rode Stereo VideoMic on my 6D mounted to the roof rack for an opening day drive on Trail Ridge Road in Rocky Mountain National Park. I left the buffeting in since that's how it would be with the top down. You can pick out a voice pretty clearly as I drive by at 0:22.Trail Ridge Road Opening 2011
(The video's owner prevents external embedding)