I was going to say gear thread but this is perfect to exemplify /p/'s gearfaggotry and why bigger apperture is not always better.
I'd recommend the 17-70 for several reasons:
1. The focal length on apsc is basically a 24-105 on full frame, a great overall lens that will cover 90% of your needs. Those 24-105 are very commonly used by professionals (35mm sensors) and are fixed f4, so you have an even brighter lens than them on the wide half.
2. The aperture is 2.8 until 23mm, then remains at f3,5 until 50mm, and then changes to f4. That is only half a stop less that the 17-50 f2.8 during the same focal length, with still an extra reach until 70mm at only f4, which is still quite fast and a great apperture for portraits or indoor sports at 70mm (perfect compromise between light gathering and depth of field).
3. It's the closest thing to a macro lens, it focuses extremely close and since the sharpness up from f5.6 is astonishing you can crop a bit and get virtually the same results you'd get with a real, twice as expensive macro prime. Remember for macro you shoot at f/values of 11 to 16 to get more depth of field focusing that close.
4. The autofocus is as good as, if not better than in the 17-50.
5. Sigma's contemporary line is known for being very solidly built and with a great overall quality. Get the newer version and you won't be disappointed, whereas the 17-50hasn't been updated yet.
A bigger apperture is always a good thing to have, but for me half a stop is not enough to compensate for the lack of macro capabilities and the extra reach of that short tele.
Pick your poison mate.
