Here's the passenger traffic according to wikipedia (not sure what their source is) at subway stations at each street near the potential DRL allignments.
Front: 95,290 (Union: 95,290)
King: 114,040 (King: 60,270, St. Andrew: 53,770)
Queen: 81,960 (Queen: 58,710, Osgoode: 23,250)
Dundas: 100,640 (Dundas: 69,150, St Patrick: 31,490)
College: 96,730 (College: 49,340, Queen's Park: 47,390)
The stations along King do have the highest ridership, but it's not that much higher than those at Queen or Dundas. Also a subway along Queen also serve the areas around King and Dundas quite well, the two streets are only a 5 minute walk from Queen. A subway along Front would not serve the areas around Queen and Dundas quite as well.
I know that the amount of passengers at subway stations is not the best way to measure what the ridership will be, but there are still quite a few destinations North of Adelaide, including a fair bit of offices, the Eaton Centre and Ryerson. Between Yonge and the Don River, a Front allignment would probably be better, but West of Spadina I think a Queen allignment would be better than Front. The areas South of the Gardiner would probably be better served by Queens Quay