Here's my rationale as to why elevated light rail (like the converted Scarborough LRT) is the way to go for the Eglinton non-tunneled portions and the Sheppard line - which is also what I think Metrolinx is trying to lead the TTC to.
LRT advocates argue that subway is too costly, canceling Transit City would cause a cancellation in the LRV contract, and that there is not enough capacity to justify a subway.
Subway advocates argue that the Transit City lines (at grade portions) are not fast enough to be able to connect the city especially on the major thoroughfares like Eglinton and Sheppard, and in Ford's case - that LRTs take away road space.
An elevated LRT would appease the LRT crowd since it would use the same vehicles that was already ordered for Transit City, and would be lower capacity and lower cost that a subway.
It would also appease some of the Subway advocates by providing true rapid transit, and take away less road space than the LRT.
Of course, this isn't a perfecto solution to the pro-TC people and the anti-TC people, but I think it would be a good compromise between coverage of the city/cost (which TC provided) and speed (which the all-subway plan would provide).