I'm comparing sulfur tetrafluoride and thionyl tetrafluoride. The lone pair has more s character compared to a double bond. Therefore, sulfur in $\ce{SF4}$ should have a greater s character compared to sulfur in $\ce{SOF4}$.
Since $\ce{SF4}$ has a greater s character it's bond length would be comparativly smaller than that of $\ce{SOF4}$.
Therefore, $\ce{X}>\ce{Y}$. But my book says otherwise. Did I go wrong anywhere?