I own both the G1X and RX100 (in addition to my 5D Mk2 and 7D). Bought the G1X as my vacation camera, but am using it a whole lot more than I thought I would, especially since it has a hotshoe mount. I picked up the RX100 mainly because I had some unused Sony Card points that brought down the price to a reasonable level. $650 is very steep in my opinion.
IMHO, the G1X is underrated by most reviewers and the RX100 is a bit over hyped (it's the flavor of the month just like the X100, NEX 7, etc.). I also feel the term "SLR like quality" is very overused. The image quality of the G1X is close to that of a 7D paired with a 24-105 lens (especially when shooting in proper light). The RX100 has very nice image quality, but the overall image feels compressed compared to G1X and will not give you L glass quality pics.
I am a very picky (probably OCD) camera user (1 part tech geek / 1 part amateur photographer). My obsession for quality vacation pics drives my wife crazy. It takes a lot to "blow me away" (another overused term these days). The RX100 has mind blowing image quality compared to an iPhone 4S. If you must have a pocketable camera (or if video is of primary concern), I would go with the RX100. However, I'm not sure what kind of vacations you plan to take. The RX100 is a city slicker and I doubt it would hold up well in humid or dusty conditions. The G1X seems more rugged to me. Other than the cracker jack viewfinder, the only fault of the G1X is sluggish autofocus speed / photo processing time. It is a bit over priced, but is quickly approaching the sub $700 range.
If I were you, I would hold off and see what the EOS M has in store. The other option is to get the 40mm pancake lens and take your 5D MK3 with you.