The 18-55 you have is quite good. But the 15-85 is noticeably a step up, and the difference between 15 and 18mm is tremendous. So if you want a wider angle, that might be the lens for you.
But instead of replacing your lens in the standard zoom range, you might opt to spend some time learning a bit more about software for editing colors, sharpness, using masks, and merging/stitching images together.
What you really need is a tele: 55-250