I'm on the fence about Air Transat. We've had good flights and bad. The bad have been very bad. Our last flight with them, last year, we waited in the check in line for 45 minutes and by the time we got to the counter they didn't even have 2 seats left together. I was travelling with my 2 kids. My older DD was 15 so it was not really a problem but my youngest was only 6. That was problem. They were completely unwilling to even try to change some seats to accomodate a child and on top of that the staff was rude about.
I have flown with them about 15 times and have never had an issue with schedules being changed or anything like that. Every problem has had to do with their staff or procedures.
If Buffalo is an option it is a far better way to go. The prices are MUCH, MUCH lower than the prices from YYZ and the park and fly is easier and cheaper. Also, the airport itself in Buffalo is very simple to get through.