Canon 17-55mm or sigma 18-35mm ?

I think I'm beginning to get it now.

1) Single lens option:

a) Entry level stock EF-S 18-55 STM, with roughly 3x zoom.

b) Wide range zoom EF-S 15-85 with roughly 6x zoom. Zoom range a little restricted because its an older model, and the extra wide 15mm end makes it harder to have a longer zoom range.

c) Wide range long zoom EF-S 18-135 STM with roughly 7x zoom, thanks to being relatively new model.

d) There is a very wide range long zoom EF-S 18-200 with roughly 11x zoom, but it may be a bit old.

All the general zooms mentioned above have an average aperture range of f/3.5 at the wide end, to f/5.6 at the long end.

Understand that the single lens option will:

a) never be that ultra wide,

b) never be that long,

c) may not be huge in aperture.

2) Multi lens option:

a) Something like an EF-S 10-22 ultra wide for width.

b) Something like an EF-S 55-250 etc for long tele.

c) Zoom lenses with wider apertures:

The zoom range diminishes, in favor of larger aperture sizes.

i) Traditionally, the EF-S 17-55 f/2.8 with roughly 3x zoom range was a zoom lens that bridged the gap to wide aperture prime lenses, with an aperture of f/2.8 that's 2/3 of a stop wider at the wide end, and a full 2 stops wider at the long end for four times greater brightness.

ii) Now enter the Sigma 18-35 f/1.8 with roughly only 2x zoom range; it is only wide-normal for focal length, and not long, but it packs a bigger aperture of f/1.8 that is one and a third of a stop wider than the EF-S 17-55 f2.8. Hence, the Sigma is trading zoom range, for aperture size!

Thus both the EF-S 17-55 f2.8 and the Sigma 18-35 f1.8 are good, but in different ways; both have their strengths and weaknesses.

For a combination of zoom range and aperture compromise, still go for EF-S 17-55 f2.8.

For outright aperture, but a narrower zoom range, and for something that is even closer to a single focal length prime lens, go for the Sigma 18-35 f1.8.

3) Single focal length prime lenses with the very largest apertures.

For a truly wide aperture, we must go to single focal length prime lenses.

However there aren't too many prime lenses dedicated to crop sensors.

4) For a truly wide range of lenses in general, and wide aperture prime lenses, we must move to full frame sensors...