IDK about a 35mm. It's too wide for most portraits and too narrow to be used as a wide... Maybe thats just my 50mm brain stuck in that view.
But if I were to buy one, It'd be the sigma 35mm. It's fantastic from what I've seen and heard.
It's all a matter of personal preference. I don't like 50mm, I find it too long for general/travel use and too short for portrait. Sure, 35mm is also too short for portrait but it's perfect for general/travel use. If I wanted a prime for landscape, it would be 24mm but for everything between landscape and portrait, I find 35mm (on FF) to be perfect whereas the 50mm is just kinda blah.
And the fact that, across all my systems, i have five 35mm primes but no 50mm primes shows I practice what I preach!
As for the OP, if I had to chose one prime to replace a standard zoom, I would pick 35mm without hesitation. Whether it's the Sigma or Canon 35L, I can't say because i have no experience with the Sigma but I love my 35L.