The Rode mics often have on camera mounts that will insulate them from camera vibration. If you have the original 6D you may not have auto focus vibration, but pan tilt from a tripod can transfer vibrations up to the mic that will be picked up as sound in the recording. The solid mount of the Canon will not insulate the mic at all, the suspension of "better" mics will dramatically reduce the sound picked up by the mic.
You also have options for dead cats etc.
I don't know about the actual sound comparison, but voice recording is not too demanding, anything will make an improvement over in camera.
PSA: The above post may contain sarcasm, reply at your own risk | Not in gear database: Auto Sears 50mm 2.0 / 3x CL-360, Nikon SB-28, SunPak auto 322 D, Minolta 20