I drive back and forth between the Niagara region and Toronto regularly... knowing how bad traffic can get on the weekends in the summer, I don't think you could fit in much more than Toronto to Niagara and back in a day.
If you decide on the shorter itinerary... leave the city early. I have no idea why, but for some reason by mid-morning, traffic crawls around Burlington and Hamilton. What should be no more than a 90 minute drive can easily extend into three hours.
As you cross the Lewiston-Queenston bridge, look to your left and give me a wave....