1. I would suggest a Saturday over a Sunday since a lot of people come from all. It really doesn't matter for the people in the GTA area or Niagara region, as the difference between GTA and Niagara is a max 2 hours. Other that that are traveling from other parts of the country, ie Ottawa, Montreal, and our American friends, might need the Sunday for traveling back.
2. Besides the attendees, I think you should consider where the majority of the dealers are coming from, if it's too far, maybe there will be less dealers willing to travel, since we presume the cost of gas will be at $2.00 a litre by next year.
3. A location that doesn't cost an arm and a leg for parking, (free is alway better). So a place that has enough parking to you don't have to stress out to find parking.
If Niagara was chosen, I would consider looking at locations away from the falls - as there isn't much parking there and what is - is pretty expensive. I wonder if areas such as Lundy's Lane would be better - I would also presume that the cost of the hotel would be much cheaper than the ones located near the falls.
If being held in Toronto, we know parking is always expensive, there is lots of traffic. Maybe a suburb of TO would be cheaper to stay and have a bit less traffic?