I just did the same thing myself. I bought a D3200 and borrowed a D5100. I also bought a Nikkor 10-24 and a 24-120, all for a three week trip to Europe. I have since sold the 10-24 and the D3200, but it was a fantastic combination. There was no way that I was going to lug my D3S and a second body as a backup. Of the two cameras, I liked the D3200 the best, but there was nothing wrong with the D5100 either. Sometimes I carried both cameras with a lens on each, and the rest of the time I carried one camera on a BlackRapid strap with the other lens in my vest pocket. I was more than pleased with the results and am still sorting through my 1,000 plus pictures.
One other thought. I just bought the D600 and if it had been available before I left, I probably would have taken that instead of borrowing the D5100 since I would not have needed to buy the 10-24 as I have a 16-35. Also, the D600 is reasonably light although certainly not as light as the D3200 and D5100.
As for the Canon S100 point and shoot, I brought a Nikon S8100 and there is no comparison in the shots. I'm sorry that I brought it since it would have forced me to take one of the other bodies on the one occassion that I decided to use it.