Just to clarify, I have the Canon 17-55 f2.8 for my 50D and the Fuji 35mm f1.4 on my X-E2. I don't have a Sigma 35 but I was close to pulling the pin on one.
The 17-55 is usable at any aperture, I didn't hesitate to use it wide open. As I say, the only real caveats were that it preferred distant subjects to close ones- 3m or less. Fine by me as I like landscape and architecture. Other than that (and it's not a big deal) it's an epic lens.
The Sigma 35mm Art is a fabulous, class leading optic at every aperture, and I think everyone here would agree. Use it at f1.4 and don't look back!
Speaking of sharpness, without wanting to sound like Ken Rockwell, it's not the whole story. I shot a friend's wedding a few years back, and my normal first phase of processing was to move out all the shots that weren't sharp into another folder. I processed and presented the sharp and pleasing ones. Fast forward three years or so and Google plus randomly decided to process (Auto Awesome) them for me today, and looking through the slideshow it gave me, some of the shots are really great, and came out of the ones I'd discarded. So I'm learning that sharpness isn't everything. Indeed, being in focus is nice, but isn't everything either.