Ideally shouldn't be an either/or scenario. The Feds can pour money into the TOR/MON rail corridor while finding solutions for the Transit Hub. It would, however, require more political will and appetite to actually want to upgrade the transit infrastructure in this country than the Fed Liberals have shown thus far.
The usefulness of a Transit Hub shouldn't be directly tied to flying trends in regards to Pearson, unless we think the people working and travelling to the lands in/around the airport will suddenly fall off a cliff. The western GTA needs a multi-modal hub in the worst way. The kitchener line goes right by Pearson in almost a perfect curve. Requiring riders headed to the Pearson area from Kitchener/Guelph to go all the way to Union(or Weston GO) for a transfer back up the same line is maddening.