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I can't say I have experience in either of these two lenses (the 18-55mm kit lens or the Sigma 17-70mm) but....
a. you're right that having that extra 15mm focal length would be handy for the travelling photographer and,
b. the 'kit' lenses by all manufacturers all have a reputation for being of a lower standard (usually less sharp) than the more expensive lenses by the same company or from people like Sigma.
The biggest difference in prices is usually the maximum aperture (f/stop), where a 'fast' lens (like f/2.8 or lower number) will allow faster shutter speeds, giving sharper pictures (less camera shake).
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