Depends on what you are photographing...
If you need high ISOs, thin DOF and lots of FPS (for sports, etc) then bring the XTi. Otherwise, the G10 would be more than good enough.
I happen to have a G9 and a XSi. I prefer my XSi for the most part but after studying the pictures, I have realized that my old G9 really gives the XSi a real run for the money at ISOs under 200. For me, the difference are almost nil. I bet if you look carefully, you will notice the same thing as I have. Do not pixel peep too much, its pointless in the end.
I am heading to Switzerland this summer and am agonizing over pretty much the same thing. Tonight, I took the G9 and took pictures in dying light. It turned out much, much better than I expected. But I like night photography so I am still agonizing...
There is one caveat: The G10, like my old G9, may benefit from being in M or Tv mode. Otherwise, the camera will tend to use too low a shutter speed which leads to soft shots. I bought my XSi after not being satsified with my G9. Then I learned manual control and found the G9 to be much better than I thought! Just mentioning this if you're not using M or Tv mode.