Recently finished processing my pics from our trip a couple of weeks ago. The S100 pics are decent, but the 5DII images (with 28-300L) are a clear step above, even at screen viewing 2 MP resolution.
FWIW, even though my wife and kids are quite tolerant of 'the big camera' I don't let it slow us down. With a 2.5 year old, we still have a stroller along (if she's tired, but also for convenient storage). When we all went on a ride, the 5DII+28-300L stayed in the Lowepro Toploader Pro 75AW tucked under the stroller (risky? maybe...but that's what insurance is for!).
Looking over the pics, I'm glad I had both cameras. The meetup with 'Cinderella' in her castle needed ISO 6400 (slow lens). The S100 flash would have worked, yes, but I'm not a fan of deer-in-headlights. Instead, I have princess-hugs with my daughters in the (electric) candle-chandelier illumination, preserving the ambience.