Having been to both places, I can tell you the advantages and disadvantages of the two cities.
Toronto: Being in Toronto, and especially studying at U of T, has its advantages and disadvantages. Located in downtown Toronto, close to the Royal Ontario Museum and Yorkville, a high-end shopping area, you will have convenient access to the subway, which means you will be able to explore other parts of the city. Considering that Toronto is one of the most multicultural cities in the world (and the
fourth best city to live in the world), you will have lots of varieties of restaurants, from Thai to Korean, Italian to Greek, and the list goes on. Not only will you be in one of Canada's major cities, you will be studying in one of Canada's best universities. Of course, being in one of the biggest cities in Canada at one of the best universities in Canada will have a bad side of it. You will be competing for marks with a lot of bright and studious students and you will be paying a lot of taxes with a high cost of living (however, in comparison to Hong Kong, it isn't much). Another bad thing about studying abroad in North America means you will be paying a lot if you are to travel to other cities. If you don't mind being in the bus for hours to get to your destination, since taking the train or plane is really expensive, then you would be able to go to cities like Ottawa, Montreal, Quebec City, New York City, even Boston. With that being said, Toronto will be a very interesting experience for you and I think you will enjoy being here.
Hong Kong: HK is totally different from Toronto. It is a really exotic city and it has a lot of perks with it. If you like shopping, then you will be able to shop without tax and have a lot of opportunities to try different kinds of restaurants. You will also be studying at HKU, which would be the best university in Hong Kong. Being in a very Western-influenced area of Hong Kong, it will not be a problem conversing with others in English. Despite being in a small city with an extensive transportation system, transportation costs will be really high as you will be relying a lot on buses and the MRT, since it charges your trip from origin to destination in terms of distance. The cost of living is also really high in HK, so be prepared to bring a sufficient amount of money with you. Due to the proximity of other countries to HK, you will be able to travel to other destinations extensively without having to pay too much for it, depending on the season and which airline you will be taking. That being said, HK is a great option if you like an exotic destination with good travel destinations close by.
Now, it is your call. Both cities are great and both have its perks and disadvantages. Hope you will be able to make a choice and let us know which one you choose!