From what I've seen, the 35L has a very slight IQ edge, and definitely less vignetting, too. Practically speaking, I think it's foolish to pick one of the two over the other based on IQ, they're too equivalent. The real difference is focal length - I set my 24-105mm to each FL for a while, and preferred the 35mm framing (found myself cropping the 24mm shots). Also, the 35L can be used for environmental portraits, whereas with the 24L you'd probably need to be close enough to result in distortion of the subject's features. Another factor is weather sealing if you need to shoot in the rain - the 24L has it, the 35L doesn't.