S5 IS replacement
Probably a good choice for Europe. I'd also consider taking the S5IS for those times when you need a longer focal length, as it's about 3x the maximum of the G1X. Plus, you have a backup if something happens to the G1X. On significant trips, I always take at least two cameras to get a wider range of capability and to have a backup.
The S5IS (I've owned a couple of them) is a very good camera with quite a fast lens (f/2.7-3.5), and the lens is quite good. Its biggest drawback is the chromatic aberration (CA) in high contrast situations. Later Canons finally got around to doing automatic in-camera removal of CA with the SX40HS. Also, you're quite right about ISO 400 being the maximum usable ISO.
If the G1X proves to be too big or heavy, than I'd recommend the G15; but replace the neck strap with a single point attachment neck cord. It makes the camera a lot easier to pocket and at 12.5 oz. it's light enough that the cord doesn't hurt your neck. With the neck cord, it does fit nicely in a looser trouser's pocket. The G15 has a slightly longer lens (140 mm Vs 120)than the G1X, and is also faster at f/2.8. ISO 800 is quite good, and with the fast lens, you can shoot in available gloom. The G1X has lower noise by over a stop, so that the lower noise offsets the slower lens.