When I was doing my research, I found most street photographers prefer the 35mm over the 50mm due to the angle of view. 63deg vs 46deg. This means that you can capture more of the subject's surrounding into the photo to relate the story. Personally I think this is a matter of individual preference. some prefer street portraits (less bg) some prefer shoot from behind the wall (70-200).
2nd factor was price. 35L is cheaper than 50L (for me anyway)
3rd factor was 50L received alot of negative reviews but after a quick Google search, seems like the newer batches have been updated? (not so sure)