I guess its worth what you're willing to pay. I paid £700 for the twilight at release. For my tastes and needs, i value this phone more than an s9+ or iphone x.
I dont game on mobile, so take that into consideration.
I prefer the design over the s9 (i dont like the edge screen). For day to day they seem to perform very similar (i wouldnt be surprised after a few months if the samsung started to lag). However, the battery life of the p20 is much better.
In 7hr tests they both seem on par. But in day to day normal use the p20 lasts a lot longer. This to me is a much bigger difference than the bump up in resolution.
Or maybe im coming from the wrong pov, and really i should be saying that no phone is worth 700, including the X and S9+. I had the choice, price wasnt a factor...