What was the decider? I wanted to have "drop caps", that cool feature where the first letter of a chapter is 3 or 4 lines big. The forums for Scrivener suggested some very complicated ways to do it involving CSS. In Vellum it's just one of the styles. Silly, but it's my book and I wanted it.
Vellum also seems to know publishing conventions better (the half-title page, the copyright page, About the Author, etc.) They make your book not look so self-published.
I'm not trashing Scrivener, though. It's a good product.