Is it better for Canadian retirees to apply for a retirement visa in Canada or Thailand?

Both are possible indeed. When you make a non imm O retirement in Thailand you have to go to immigration a couple of times and already transfer 800k฿ in your Thai bank account as international transfer. Then after 60 days you can ask for a year extension based on retirement. With the non imm O retirement in y'r country, you get 90 days already and after 60 days you can ask the year extension with or 2 months before the 800k฿ in your bank account OR an income statement (affidavit). Much less hassle.