Just as an aside, where is the ridership in western Toronto that the DRL is supposed to intercept? What destinations do we want the western leg to reach?
The obvious answer is Jane Street. Between the 35 Jane and 195 Jane Rocket buses, there is combined ridership of 43,500. However, the Jane corridor is about to be intercepted in two locations, at Finch and at Eglinton with the FWLRT and the ECLRT respectively. We will have to wait and see how commuters adjust to these services, but my presumption will be that while overall ridership remains high, peak point ridership will remain below bus capacity. Further, there are not too many destinations on Jane Street itself, aside from the obvious Jane&Finch community which is getting the FWLRT.
Beyond Jane, these are the other options and their current bus routes:
Dufferin - 44,000
Landsdowne/Caledonia - 15,400
Dundas West/Junction - 4,900
Keele - 24,100
Weston - 15,200
Scarlett/Runnymede - 12,300 combined
Royal York - 8,900
There is an obvious front-runner here with Dufferin. Even more so the case when the highest ridership routes besides Dufferin (Landsdowne/Caledonia, Keele and Weston) will have the northern halves of their ridership intercepted at Eglinton, and others (Scarlett and Runnymede) being more of 'coverage' services that will probably be feeding into Mt. Dennis too.
Dufferin meanwhile, has the majority of its ridership generated south of Eglinton, and has multiple major destinations (Little Portugal, Dufferin Mall, Galleria Mall Redevelopment, Corso Italia and Fairbank (as well as Yorkdale Mall, if extended that far north)).
I believe that if not Dufferin, then the Relief Line should go to South Etobicoke instead. Creating a multi-modal hub at Sunnyside/Roncesvalles Carhouse would be very practical for the overall network, and the density at Humber Bay Shores is more than enough to support higher order transit.