I just dealt with this question myself.
From a performance standpoint, I think Canon's 24mm L II is considered the better of the two lenses.
Focal length is a personal thing. For me, 24mm is too wide for a general purpose walkaround. And in my bag, it would get paired with a 50mm. But lately, I've been thinking that a 35mm + 85mm combo will give me optimal flexibility/quality for the way I shoot. I feel like most lens combos revolve around 24/50/100 or 35/85/135. But obviously, this is a super personal thing and I can think of situations for shooting every focal length.
I bought a refurbished 35mm f/1.4 L, but was underwhelmed by that particular copy and returned it. And so now I am considering the new Sigma 35mm f/1.4 or trying a new Canon 35mm f/1.4 (rather than a refurb).