Ask anybody who has lived in a city that has hosted an Olympics or Expo. I have friends in Sydney, Montreal, Atlanta and Vancouver. Of the 4 people I know from Sydney and Atlanta, all felt very excited to win the olympics, but afterward, everyones opinions had changed, from mildly let down, to completly depressesd, to seriously pissed off. There is a build up of years, then in 2 weeks it's all over, everyone goes home and most people are like "was that it?". Almost nobody got tickets to any major events they wanted to see. People were restricted in where they went, not even able to get close to the Olympic cite and traffic was a nightmare. It seems to me the Olympics have little pleasure for the little guy who has no connections.
On the other hand, all my friends who lived in cities who had World's Fairs, told me it was amazing. First of all, if you get the season pass you can go enough times to see everything. Anyone (who can afford the moderate price) can get tickets. There is a buzz of people from all around the world for half a year and at a world's Fair it just seems that people mix and it's easier to meet people. World's Fairs seem to be about a celabration everyone is invited to, a real party for the city, it's people and the world, where an Olympics is quite exclusive, short lived and intrusive.
As far as I can tell, from the people who have experienced it, it seems a World's Fair is a much better bet. If it leaves behind iconic attractions and buildings, it will have an even greater legacy. A World's Fair done right is amazing, a World's fair done wrong, on the Cheap, can leave a disasterous impression of Toronto on the world, for years to come. Just ask Knoxville!