S5 IS replacement
The G1X Mark II is a significant camera for significant money. The lens offers a much better aperture range through the zoom. The S120 significantly smaller. Turned off, the S120 is almost the size of just lens on the G1X Mark II.
So, this is really a question of: how small a camera do you really need? What compromises are you willing to make to get to that size? And what is your budget?