Sketch. UXPin, like Mockflow is good in theory...But you get better speed/performance from Sketch. And flexibility.
That said, the big attraction with UXPin, Mockflow, etc. (and there's been oh so many over the years) is the ability to click through. It gives the sense of a "prototype" even if it's not a very sophisticated one.
One could very easily create some HTML and link images and such (and we used to do this with image maps wayyy back in the day). Though that takes time.
So the best option I've now seen is exporting from Sketch into an online tool. At that rate UXPin and Mockflow, etc. seem overkill. They are still quite viable and work with Sketch (whole page comps or elements exported into those tools) quite well.
Less of an either or. More of a both.
I like InVision because it has a free plan. I wish UXPin did. At the end of the day InVision + Sketch is my preference.