EFS 17-55 vs 15-85 in low light

Hi, I know a lot has been said about these two lenses. The 17-55 has faster apertures BUT the 15-85 supposedly has a better IS which should give it one stop advantage (4 vs 3). So is the 17-55 any better in low light? Eg at 18mm the 17-55 has 2/3 of a stop advantage but the 15-85 has the one stop IS advantage so is the 15-85 better in low light at this focal distance? At 55mm the net difference would be one stop in favor of the 17-55 (2.8 vs 5.6, so 2 stops minus the one stop IS advantage of the 15-85).

I shoot quite a bit in low light (travel/street photography) so thought initially the 17-55 would be better, on the other hand I saw that I shoot more than half of my pics at both end of the 18-55 lens so the extra zoom and wide angle of the 15-85 might be useful.