Sigma 17-50 f/2.8 vs Sigma 17-70 F/2.8

I have the Sigma 17-50mm f/2.8 OS HSM, which is calibrated for my camera.
I haven't used the 17-70, but 17-50 is an okay lens.
The OS isn't really 4 stops as it proclaims—more like 3 stops to my eyes.

The constant f/2.8 is a nice feature, but sometimes, 50mm feels a little short on the tele side (of course, you can swap your lens with a telephoto zoom, but when traveling and this is your only lens, it can be frustrating).

I don't know if my copy is a dud, but it looks a wee bit soft.

I did a crude focus testing, but the lens was in perfect focus, so maybe I have an inferior copy xP

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My current arsenal:
Canon Rebel XT
Canon 50mm f/1.8 prime
Canon 70-200mm f/2.8L IS USM
Canon 100mm f/2.8L Macro IS USM
Sigma 17-50mm f/2.8 EX OS HSM
Panasonic Lumix DMC-GF2
Panasonic 14mm f/2.5 G Aspherical Lens