Well, to me, this is a focal length issue. I do think at f/1.2 the 50L is sharper than the 35 at f/1.4. But as you stop the 50L down, it gets less sharp, and actuallly gets overtaken by both the 50 f/1.4 and 50 f/1.8 beyond f/5 or so. However, I speak in relative terms, as the 50L is still sharp at all apertures. It simply is sharpest from 1.2 to 2 and after that there are other lenses sharper. The 35L is not as sharp in my experience as the 24-70L II at all apertures. Howver, and again, this is relative, as the 35L is sharp at all apertures. So it boils down to focal length needs. I would not do a big family photo at Christmas indoors with a 35L as distortion will likely play a factor, as would any portrait shot close up. The 50 would be better for this. So I guess either is great, you just need to decide on focal length. I didn't feel the 50L was worth the huge price premium, so I have the 50 f/1.4 and I'm gettting every shot I could/did than when I had the 50L. Of course though, I did pick up a 24-70L II zoom lens and now don't use either prime, as I feel on my camera bodies the zoom lens, f/2.8 and narrower, is superior to either lens.
Bottom line is neither is a bad lens and you will be really happy with either! Just make sure you pick the right focal length for your needs.