The 17-50mm f2.8 Sigma offers you increased maximum aperture along with a constant maximum aperture. That gives you more light gathering ability which is helpful in lower light conditions (for both exposure and viewfinder image and AF reliability). The constant f2.8 also means that you won't find the aperture shifting as you zoom on the lens.
It is likely that in a higher level of lens and with less focal length coverage it will also be sharper when you compare it wide open (max aperture) to your current 18-135mm
The 18-135mm really only one bonus in the 50-135mm range it covers.
Whilst focal length values between extremes on a zoom lens are not always exact, dry your zoom in the 18-50mm range; see if its suitable and what you like to use a lot of the time. If you find that its really not where you're using it all that often then its telling you that, whilst the 17-50mm is a good lens, its not going to give you something that you "want/need". On the flipside if those are focal ranges that you're focusing on in a big way then upgrading would make more sense.
