At this point, what I'd like to see most is the SRT just get rolled into the implementation of GO REX. Chances are the Stouffville line will be electrified before the subway is ready to go anyway. If you're looking at similar timelines, I think GO REX is the better, and more cost effective, option.
It also solves the problem of being seen as "reverting" back to the LRT. The support for Tory's SmartTrack in the general public has been pretty positive. The Scarborough GO REX could be marketed the same way, as an express above ground subway. Subway advocates are happy because as far as they're concerned it's a subway. LRT advocates are happy because it would follow the same alignment as the SRT + extension, and would cost about the same.
As an interim solution (while the SRT is shut down), get GO to install an extra track between Kennedy and Agincourt, and run a smaller GO train set between the two, with a temporary station at Ellesmere. It would be accessible with a TTC fare, and would drastically reduce the number of shuttle buses needed while the SRT is shut down. Basically the same type of implementation as the O-Train pilot project, only using existing GO rolling stock.