Looking at your last photo comparison, the 55-250 looks a bit sharper. The colours and overall look of the 15-85 looks better to me. The 55-250 maybe looks slightly over exposed while the 15-85 slightly under. It depends what you want, since the house has strong light on it but other areas have shadows on them. It looks like it gets more light than your 15-85 in the same settings. Maybe you should also compare other parts of the photo in 100%, like the sides/top/bottom and parts with a different distance from the camera (e.g. the front of the porch on the left side).
So are you sure the difference is not a focus issue? Maybe you didn't manually focus exactly as good with the 15-85? As far as the shadow being different etc. that someone mentioned, that could have been wind moving the trees and plants plus light can change pretty fast sometimes (though I didn't check this very thoroughly).
I agree with the the above poster who said the 55-250, although not underpriced IMO (I don't think any lens is underpriced), can be a very good lens, at least in part of the range, especially for the price. I also agree the 15-85, although great (my favorite walk-around on a x1.6 body), is a bit overpriced.
It's like an 800m race between Usain Bolt and the 800m champion. I'm sure Bolt would be very good and beat almost everyone, but not the 800m champion. So the 15-85 isn't the Bolt of the lens world and the 55-250 isn't the 800m champion, but the same idea. Compare them at 200m and the 800m champion didn't even start by the time Bolt finished...
Just because the 15-85 is much more expensive doesn't mean it will be sharper than the 55-250 at all overlapping focus ranges. IMO it also doesn't really matter if many think your lens is ok or not. Most important is that you are happy with it. If this result (regardless of whether yours is defective or not) is not good enough for you, then probably best to return if you can. Maybe try another copy to check. Many complain about quality control issues with this lens.
I don't have the 55-250 for comparison but it's very hard to say. Based on that 100% crop I'm leaning to your 15-85 photo having a small issue. I can't be sure and I'm emphasizing photo and not lens because I can't say the problem is with the lens either.
I just compared my 15-85 at 61mm (couldn't get it to 60mm) with my 60mm macro which is very sharp, both at f/5 & 1/125. For some reason the objects (which were about 15m-20m from me) in the 60mm photos came out slightly bigger, maybe I held the camera a tiny bit closer, I'm not sure. In sharpness there is a small difference but far from huge. The 15-85 is also slightly less exposed. But I don't know if the 55-250 at 55mm would be even sharper than both of them, but I doubt it. My tests show me that the 15-85 is very good for a walk-around zoom, being close to a very sharp prime sometimes. But your 100% does look a bit strange, possibly an issue.
Re the zoom creep, it almost never happens with this lens at the ends of the zoom. That's why some people (not everyone) claim they don't have it when they do. The creep is usually between 24mm and 50mm only, when the camera is pointing close to or exactly up or down. To check, put it at about 40mm and point up or put it at around 35mm and point down.
Here is my comparison.