I own the 35L and have used the 50L several times. The last time was just a few days ago. Here is a shot with the 50L on FFF, wide open:
I happen to have a very similar shot, taken a day or two before, with the 35L:
They are both excellent lenses. The 35L is sharper wide open but this would be the last consideration for me. They both focus well. The 35L can show double lines in the bokeh under the right (wrong) conditions. The 50L can render busy bokeh as well but rarely (you can see some evidence of that above). I would chose based on the FL length only. 50mm is boring to me, 35L is just perfect. For event shooting, 50mm might be better though.
35mm sometimes need to be corrected for geometric "distortions". The head of the cop on the 2nd shot is stretched. I can use DXO Optics Pro to compress the borders mainly. It works quite well.