Canon 16-35 f/4 vs. Sig 18-35 f/1.8 Art?

[LovePhotography]

Don't hear much about the Sigma 18-35 Art lens...
I've got the Canon 8-15mm and the Sigma 24-105 Art (which I really like)
Looking to fill that gap for my 6D and 7Dii bodies.
The Canon has the extra width which I admit I would miss. But the Sigma is f/1.8 and $200 less, and, to be honest, despite all the hoopla about how sharp the Canon is, the Sigma seems just about as sharp, if not sharper based on this comparison http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=949&Camera=736&Sample=0&FLI=0&API=0&LensComp=854&CameraComp=736&SampleComp=0&FLIComp=0&APIComp=0
Although, granted, the test is not run on a full frame body...
I mostly shoot still life, landscape, but some real estate, general enthusiast stuff.
Anybody ever tried both of them?
Thoughts?

 

[LovePhotography]

Crap.
The Sigma only APS-C.
Never mind.
Okay, the Tamron 15-30 then?

 

[PhotographyFirst]

Both are going to be plenty sharp. Just pick whether you need f4 or f2.8.

f2.8 is pretty critical for doing any sort of nighttime sky landscapes, but other than that, f4 is fine for everything else.

 

[rs]

I recently made the choice between the Tamron 15-30 and Canon 16-35/4. While the Tamron is faster and wider, what put me off were its higher price, heavier weight, it can't take filters, zoom/focus rings which turn the wrong way, and a niggling thought in the back of my mind that there might be compatibility issues further down the road.

 

[Hjalmarg1]

I'd pick the Canon because:

• has wider focal range
  can take filters
  it's weather sealed
  works in both FF and APS-C

Unless you want to capture action fast moving objects where f1.8 has a greater value.