Should I get the 17-55mm IS USM lens?

i used to have the 17-55 and only got rid of it because I got a 6D and 24-70. I just got the 18-135 to go with a SL1 for a general purpose all in one type kit. The 17-55 is a superb lens. Image quality is great so for pixel peeping it will always win out. I've only had the 18-135 a while, but it seems to have above average sharpness for a lens with a wide FL range. It focuses really fast and the IS is great and I believe better than my 17-55.

In short the 17-55 will give you some lower light capabilities due to its faster aperature, slightly better image quality that I doubt will show up in most real world photos, and less DOF if you want that. The 18-135 gives you a great focal length range for a walkabout not sure what I am going to shoot type lens, it has better AF and IS. If you ever use live view the STM motor will work noticabely better with the SL1, 70D and newer Canon bodies.

I have always recommended the 17-55 to people wanting the best general purpose zoom lens for crop cameras, but after getting the 18-135 I can see where for a lot of people it may be the better lens depending on their needs.

You have a great lens now. Getting a 17-55 will give you another great lens with a different set of capabilities.

Edit:  Just realized you didn't state what 18-135 you had.  My comments reference the newer STM version.  Never had the other version, but I from what I read older one had good optics that were close if maybe not totally the equal of the new STM.  Hope this helps.