I agree. I'd reread the previews of both and looked to see which were on audiobook (both were). I sampled and liked both narrators. Both goodreads ratings were about the same. I considered page count but even though The Immortalists is longer, Moon Tiger seems like a slightly more complex book so I feel like their reading times would even out more in the end. I thought it'd be nice to read such a recent book as the Benjamin, but then it'd also be nice to read a Booker winner like the Lively. There was nothing to find to tip the scales. I was going back and forth so I'm happy you took the weight off my shoulders here, as it were.
I'll also vote for Moon Tiger. I read some of its reviews on goodreads while trying to decide and I liked some of the reviewers' thoughts about the book and time:
'Utterly compelling historical novel that plays with time and perspective in fascinating ways.' ; 'Reading Moon Tiger is a catalyst for drifting into one’s own memories.' ; 'The way Lively used language blew me away. I can't compare it to anything else I'd read before.'