It's certainly cheaper to get the Canon kit lens, if that's all you want. If you also really want the longer lens, then the Sigma kit may be cheaper. But it's not just about cost. When you get into the longer lenses, you start needing to ask yourself whether you want image stabilization and wider apertures (big $$$, heavy lenses) or whether you're concerned more about travel weight.

I'm currently sitting on the fence about a longer lens because I haven't found a reason that I might actually need to use one. Since the long lens I'd really like (the Nikon 70-200 VR) is $1500 and is both large and heavy, I'm in no hurry to buy it. My next lens purchase is more likely to be a 50mm f/1.4 prime or maybe one of the ultra wideangle options (Nikon 12-24 DX or the 10.5 DX fisheye).