I agree with the previous posters. Both apps are great but take completely different approaches to the writing process. Rather than gamble on either, I'd recommend testing them out if possible.
If you have access to macOS, I'd recommend giving their trial versions a shot. The iOS version of Ulysses is identical to the macOS version, and while Scrivener has fairly significant differences between the two versions, the underlying philosophy is the same.
It would be easy for me to make a recommendation based on what I like, and I could lay out the fundamentals of each, but I think you'd be better off to take each for a test drive. At the very least, as someone else mentioned, you could watch some videos of them in action.
Happy writing!