For comparison, I did move to Canon from Sony, and their Zeiss 16-80 f/3.5-4.5 was nice. It's not constant aperture either, but that bit more aperture on the long end did help at times.
So, why fixed aperture? Why not even faster variable on the wide end like the Sigma 17-70mm f/2.8-4?
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 15-85, TS-E 24, MP-E 65, Zeiss 50/2 macro, Sigma 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye