The only logical choice from anyone who has played Xenoblade before and FF before is Xenoblade.
I mean come on, more than 5x the size! With the importance of graphics in FF, and the way the series has always prioritised that over large world's, I doubt that FFXV will hit even the size of the original Xenoblade, let alone 5x that.
Similarly ofcourse it all depends on variety. We've seen a bit of variety in X, but we definitely need to see more.