The learning curve for Apollo is a bit shallower for two reasons: (1) they have GraphQL libraries that allow you to implement a Relay-like client on top of an existing GraphQL end point (Relay requires some custom fields); and (2) better documentation.

We went with Relay, however, because it is, in my experience, more robust and more polished. The way that Relay handles fragment composition and defers component rendering, along with data masking, is much nicer and more... holistically considered(again, in my opinion). Using Apollo can feel a bit like patchwork sometimes.

Plus, ultimately, Relay is backed by Facebook and used in Facebook applications, in production, so it's not going anywhere.