I would - just because economically it is more symbiotic with Toronto, and it will probably be part of the commutershed of the central city in the future.
AoD
It is certainly symbiotic, but I think that KWC still sees itself as an adjacent metro region to the GTA, and not really part of it. Personally, I think the boundaries for the new Province should be more or less the outer boundary of the Greenbelt (though I'd remove northern Durham Region and the escarpment area north of Caledon):
http://www.mah.gov.on.ca/AssetFactory.aspx?did=17124