I support the subway for a couple of reasons:
1. I am of the opinion that transit should be built correctly, not just the cheapest possible way. This means that LRT works on corridors such as Finch, Eglinton, and Sheppard, where ridership is low and works well with LRT but would be too low to justify a subway. The Scarborough subway meanwhile will get roughly 2.5 times the ridership of Eglinton, and 4 times that of Sheppard. The level of demand on the corridor, while it could work with LRT, also completely justifies a subway, and actually beats DRL ridership projections by a small amount.
2. It makes the Sheppard LRT (which I fully support) a whole lot more useful. Previously SELRT users would have had to ride the LRT to don mills, transfer to the sheppard subway, and transfer again to Yonge. The SRT connection to the SELRT was too far east and proved to be little worth, while the subway connection is right in the middle of the LRT line meaning a faster commute for SELRT riders.
3. Time savings. While the physical speed of the trains for the SRT LRT isn't much different than a subway, there is an additional transfer, more stations, and some tight curves that the trains need to slow down for. The subway extension will cut 7-10 minutes off of the travel time to STC.
4. no SRT closure. for the LRT the SRT would have been closed for 3-4 years, roughly from 2016 to 2019. This makes a TTC commute for Scarborough utter hell, as bad as it currently is. It would have a major effect on Scarboroughs economy as well, as access to it would become much more difficult. GO buses would no longer stop at STC as there would be no TTC connection, etc. commutes would skyrocket by 15-20 minutes for the time. Scarborough is already the least accessable part of the city, getting rid of its transit connection for half a decade isn't a good idea.