I have the 17-40 and it's a very good lens overall... No regrets... Especially with the 5d3, you dont have to fret raising ISO if you need that speed... Granted, for how i shoot, for architecture, aviation, landscape, anywhere when I'd be needing the UW for my FF, I would use a tripod or monopod, I have rarely been in situations where I needed the 2.8. Dont sweat the small stuff unless you plan on handholding this lens/camera in dark warehouses day in and day out.