Christopher Hume, also using an east of Yonge definition, describes the east side as "red hot", but also notes that this development isn't at the expense of the west side:
http://www.thestar.com/news/gta/2014/08/15/east_end_toronto_a_developers_dream_hume.html
I generally go with the commonly accepted old Toronto west of Bathurst definition for the west end and old Toronto east of the Don for the east end. Though I do sometimes distinguish between the eastern (i.e. Cabbagetown, St. Lawrence, Regent Park) and western parts (condoland, Entertainment Distirict, U of T) of downtown since they do have quite different feels. That being said, I don't think Yonge St. represents that much of a divide and it seems silly to think of say, Bay St., as the "west side."
One thing to note as well is that though east of the Don developed later than the west end did, the oldest part of the city (literally Old Town Toronto) is east of Yonge and the original city center (and parliament buildings) was around Parliament St. and then the city center shifted westward. Thus, the shift into the west end feels more gradual, while the shift into the east end is a sharper one.