It really depends on you, and to some extent how long you are going to YYZ for.

CX826 offers you most of the day at least up until lunchtime in HKG. When you arrive, by the time you get to your place of residence in YYZ it will be time to sleep. So no day will be had in YYZ.

CX828 requires you to wake up early and head straight to the airport for the flight. However it arrives in YYZ at lunchtime. By the time you get to your place of residence it is early afternoon. You can slowly unwine, go out for a while etc... then sleep. So you get half the day in Toronto.

Personally I mix between the two, depending on whether I want to have most of the travelling day in Toronto or in Hong Kong. I guess CX826 is better in that you will more likely stay up a bit later on the first day.... but then you run into staying up too late because its the am back in HK. It really depends on how you deal with your jetlag.

Certainly seems to be that CX826 is more popular as CX are cutting frequency on CX828. That maybe because CX827 (the return flight) doesnt do so well though. Also CX826 might be more popular because it allows people to connect and come in from different places other than HK.