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If I was making the decision I'd need to be convinced that the 18-250 would give me the same quality of result that the 17-70 will give.
Right off the bat, and generally speaking the higher the magnification of a zoom lens the lower the quality when compared to a lens with a lower magnification because there are more elements to the lens, the light has to travel through more bits of glass and the more work the light has to do to get to the sensor the lesser the quality of that light by the time it hits the sensor.
You have the 17 - 70 which has a magnification of about 4 times whereas the 18-250 has a magnification of 14 times. I would guess that the 17-70 produces better shots than the other lens. Though I don't know either of these lenses to compare them against each other I do have a sigma 18-55 and a 50-500 and the 18-55 produces cleaner, crisper shots.
So it depends on a comparison of the end result for both lenses added to the need for a single or twin lens setup.
personally, I'd probably go for the better standard lens and then get a bigger zoom lens later as, depending on what type of photography you do most of course, you will probably use the standard lens much more than the big zoom lens.
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