Depends on what you want to buy... Smoke samon, ice wine are good. Avent baby stuff is cheaper than HK I think. But you have to add 15% sale taxes for all products, except baby products only 7%.
May in Toronto is nice. Sunny, not very cold, not very hot. Around 18 to 20 C, remember to bring a jacket for evening is quite cool, 12 C.
Gap is on sale now... not sure in May. But, you can always find a little section having last season clothes which are discounted.
Tony & Tina, MAC, Biotherm, Olay are cheaper here. You can buy Biotherm and Olay from pharmacy. If you go to Richmond Hill Time Squares Sophies, you don't pay taxes for cosmetic. It seems Anna Sui and Tony & Tina are cheaper there.
The same reminder, you have to add 15% sale taxes and the high Canadian $$ now... better find out what you like in Hk and mark down the price and compare here. :)