I don't think it's worth playing "airplane lotto" for a 5-10% chance of getting lucky if there's any significant inconvenience involved. It's not like flying LH old F is like being in LH Y. Given that the fleet conversion times are completely glacial on LH (anticipated at 5 years), that's maybe what you are looking at- maybe a handful of planes converted.

As such, unless the difference in airplane timing is trivial, do the routing that's most convenient. If you don't really care and think it's worth a shot, take the routing that maximizes your chances- just realize they are quite small.