Interesting discussion you've started. xD
Imo, it would depend upon what these words refer to.
Art has various aspects to it, so these questions come to mind when using those words.
Is the style unique? Is the way they colored it unique?
Are the characters original, or do they seem cut+paste from popular ideas?
Personally I don't really like the word "original." That refers to something as being the initial source--kind of what you were saying in your own description for it. Nothing is original anymore, but it can be unique. Like unique takes on stories that exist, or a unique interpretation of Lyndecker in someone's coloring style.
But if people use the word original to describe something, they most likely mean it as a compliment. So even though I don't use that word too much to describe others' art work, I believe most use it interchangeably with unique.