I've taken the TTC bus to YYZ many times:
TTC Service to Pearson International Airport
I've taken the 'airport rocket' bus from Terminal 1 and 3 to Kipling Station, and from there hoped onto the TTC. You could take the TTC to Union Station, closest station to your hotel.
But I concur the airport bus is a lot more convenient. If there are a few of you could could check out a one way rental to/from Pearson (drop car off in the city, pick up a car on your way back), but this seems a hassle.
Airport Bus.
Second choice: airport limo into the city, the fares are set depending on where you are going in the GTA.