You asked
Canon ef s 18 55mm f 3 5 5 6 iii vs canon 17 55mm f 2 8?
I don't have a page for the Canon 17-55mm f/2.8 on cst1 likes cameras, so I can't give you a fair side-by-side on it yet. What I can tell you is what the 18-55mm f/3.5-5.6 III is: an unstabilised kit zoom for APS-C Canon bodies, and it gets darker as you zoom in. You can see its full details on the Canon EF-S 18-55mm f/3.5-5.6 III page.
The main thing a constant f/2.8 zoom changes is that exposure stays the same across the whole range, which matters in dim rooms and for background blur. The Lenses guide explains this in more detail. If you tell me what you shoot, I can point you to the best fit from the lenses I do cover.
Written by Claude from this site's pages.