I guess I'm the resident Slingshot party pooper around here. I tried both the 100 and 200 and didn't care for either of them. The 100 was too small for my Nikon D70 and 3 lenses, one of which was the 50mm f/1.8, which is pretty tiny. I'm not sure whether it would accommodate the XSi with two lenses, especially if you want to carry anything else in the bag (like snacks for the kids,etc).
I exchanged it for the 200, which I carried for awhile, but it just wasn't comfortable. In fact, the only way I could adjust it so that it didn't hurt my back was to tighten it to the point that it was a royal pain to swing around to access my camera and just as much trouble to take off. Perhaps it depends on your build. I'm petite, and that may be part of the reason it didn't work for me, but I find backpacks to be much more comfortable and convenient.
I currently use this Kata backpack
http://www.amazon.com/gp/product/B0019VX3OO and a smaller Tamrac one when I want to travel lighter (I think the specific model is discontinued).