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To be more accurate, they are the Sigma 17-70mm f2.8-4.5 and Sigma 24-70mm f2.8 IF EX DG HSM.
There's a lot of difference. The 24-70 is a fixed f/2.8, the 17-70 is an f/2.8-4.5. Thats most of the cost. The 24-70 also has the HSM motor for faster and quiter AF, it's internally focusing so it doesn't rotate or extend when it focuses and it uses better quality glass.
You can look up what all the IF, EX etc stuff means on the Sigma website...
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