For me, I think Bloor to St. Clair (or the Beltline) is midtown and beyond that is North Toronto (synonymous with uptown). My reasons are as follows:
- North of St. Clair was the last part of the Old City to develop, mostly after 1920
- It's north of anything else in the old city (St. Clair is basically the limit for the west end)
- Beyond the big apartment clusters, Davisville (the biggest neighborhood in this area) seems to have more in common with neighborhoods north of Eglinton (like Bedford Park) than anything south of St. Clair
- Most people do have a general sense of what you mean by "North Toronto", even as these old identities do fade away
However I can see why the St. Clair to Eglinton zone is seen as something different than "core" North Toronto north of Eglinton though. It's definitely more urban.
If "midtown" is supposed to be a section rather than a neighborhood and Yonge and Davisville or Eglinton is supposed to be the heart of it, then what is the area between "downtown" and "midtown" called? Few think of Summerhill, Rathnelly etc. as being "downtown."