Lachy, the technical term (at least around here) for the line is "shadow." Or, as some would write, the "dreaded shadow."
For guys that have dark hair it's difficult to get rid of the shadow. Of course, a close shave versus stubble will make a difference. But the shadow will reappear fairly soon.
Getting some color (like a bit of tan) will minimize the distinction, but it will still appear.
As for bald or stubble ... well, totally unrelated to the shadow question, the difference in feel between them is incredible.
My suggestion ... shave all the way down so you can see how that effects the visibility of the shadow AND you'll have the bonus of feeling a slick done.
If you don't like it, you know it will be back to "stubble" length within a few days.
BTW, it really is a good look for you.