The RTX is worth it 100% over a 1080. Unless you can find a 1080 at a really low price, go with the 2080. Just for "future-proofing", and in general they are better cards. I just upgraded from my 1080, and I did notice a hell of a difference.
As everyone here has said, it depends on Budget and for what reason you're using it. For Price, GTX is easier because its older For Performance, go RTX, its newer, and has room to Improve