King would be a lousy choice. PATH is to elaborate so it would be a technical nightmare, cost a king's ransom, and the tunnels would be 10 feet short of China to make it all possible. Also King will already have ST /RER/GO so it already has the best transit connections in the city.
Queen is the natural alternative not only because it would more new riders but Queen is a more important commercial and pedestrian strip than King. Also Queen would be the easiest to build under. The biggest problem when building underground is interchange stations with other subway lines and Queen will be as easy as it gets as opposed to any other downtown street. The Queen underground station is there and yes it would have to be completely redesigned and expanded but it is still there. It would get rid of the most of the tunnelling under Yonge station.
Due to Queen station, Osgood was also designed with a Queen subway in mind. The assumption was made that it would be the crosstown route and hence Osgood station has all clearances for plumbing, sewer, electrical etc for the eventuality of a station and this is a bonus that other stations don't have. Queen's PATH system is relatively small and I think there should be subway access to City Hall.
To me Queen is not only the superior choice but also the easiest, less disruptive, and cheapest to boot.