[Klaus]

For 4K video, sharpness is pretty much irrelevant. I'd say that all modern lenses can handle 8 megapixels on full frame.

So the question is more around the soft factors.

For vlogging, I'd prefer a faster lens for a shallower DoF.

I haven't tested the Sigma yet, but the Samyang is pretty good (if you get your hands on a good sample - that's always the disclaimer with Samyang).

If you'd like to step up your game, there's the Viltrox 16mm f/1.8. But yes, it's quite a bit heavier.