I’m using the last version without subscription (16.2.6.7), so I can’t say anything about 16.6. But I see the main differences:
Vertical, horizontal or both at once can be corrected by two or four lines, each with two handles. The handles could be even more helpful, if Crapture One devs would have considered to let them become a loupe, when the cursor is on them, but maybe the newer versions are improved.
This makes the process easy to understand, compared to the sliders of Nitro. I simply adjust to line to an object which needs to be upright or horizontally Also, I don’t need to level the image before I can try to correct the perspective. And thanks to the lines, I can use small portions of the image and still get a good result. Of course, I’m used to it and maybe need more time to get used to Nitro’s way. But if I have to adjust more than one image, I have to scroll down quite a bit to get to the tools, still need more time and get less convincing results.
The 4 point adjustment of C1 (and Nitro, too) is good to quickly get a frame of a painting, a window or a monitor screen in an image rectangular. But an image of houses in an alley is different, here I only need verticals.
As @JillJam stated, the auto-correction is a very good starting point and thanks to the guiding lines (?) adjustment of facades with old houses with not much straight or upright lines still can be done.