For hand held, low light, candid shots, my favorite is the 35 f/1.4L on a full frame. 85 f/1.2L second for when you can use the reach, less in the background, etc, but because of the field of view and the required greater shutter speed, I think the 35mm is safer. 35mm is wide, but if you'r not really close to anything, not distractingly wide.


A f/2.8 zoom will be more versatile in terms of having more focal length range, and overall, far more useful across most situations, but we're talking 1/4 the light collection, 2 stops, which in my opinion is a significant disadvantage, even with IS, if you're using entirely ambient light, simply because most subjects won't be freezing so that you could take their picture.


That said, I guess it's also a question of how low the light really is. With the ISO cranked, I've taken pictures indoors at f/2.8 that were too fast and the curtains picked up the flicker of the florescent lights.


A cheap fast prime and a nice zoom might be the best combination.