Two very different cameras. Completely different from one another and incomparable.
However, the EOS-M is ONLY as good as the lens used on it.
The G1X is a self-contained unit which has a lens that is truely well ranged. It is also a complete camera with all the features on it that the EOS-M is lacking... this includes a Flip-LCD, built-in flash, an OVF etc...
So if you rwally want to compare them, the G1X is a complete camera and the EOS-M is not because it lacks numerous features that are on the G1X. To get the same lens range in the EOS-M will cost you much more money to find a suitable lens and the eventual weight difference will make the EOS-M heavier.
Disadvantages of the EOS-M include an exposed sensor when changing or removing lenses, no built-in flash, a sometimes unresponsive touch screen (which requires sticky or dirty fingers to come in contact with it to operate the camera)... etc.
Which is better? The G1X is. Even if you find a comparable lens for the EOS-M, the G1X will still be less expensive, lighter, smaller, and more complete. Even with a VERY small f/2 lens on it, the EOS-M is the same size as the G1X.