Reasons to buy the DA 18-135
The 18-135 has excellent centre sharpness through it's whole range 18-135 ƒ5.6
The Sigma 17-70 has "very good " center sharpness after about 50mm. It's not soft but it's not excellent.
The 18-135 has twice the focal range.
At 24mm the 18-135 is close to tied with the best available with excellent edge sharpness.
The 17-70 doesn't have excellent edge sharpness anywhere in it's range, but has very good edge sharpness through it's range.. the 18-135 is softer in the edges in the long end of it's range, most of which the 17-70 doesn't even cover. It's hard to criticize a lens for being weak in an FL another lens doesn't even have.
The 18-135 is Water Resistant.
Reasons to buy the Sigma 17-70
Better edge to edge consistency, not as centre sharp, but not as much difference between centre and edges.
A half stop better low light performance.
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If you look at what you give up for what you get I think you can assume the 17-70 is better if you shoot in doors in low light. Outdoors, the DA 18-135 single super zoom is simply the best there is unless you are shooting for narrow DoF.
Taken with a 17-70 this image would be half the size.. and pretty much half the resolution. Does it matter that the edges are soft? hardly, it probably is a plus in that it makes for softer more pleasing bokeh.
At 28 mm it's as good as it gets.. you can't do much better. At this focal length and @ ƒ5.6 andƒ8, better edge to edge than the 17-70 and better centre sharpness.
IN the long end, fairly close focusing for pseudo macros.
Safe to use out in the weather or where splash and spray might occur.
Good contrast , colour and flare control.
I understand people like their 17-70s.. I'm sure it's great lens, but there is a lot it can't do, if you're just taking one lens, and you're shooting mostly out doors.