At Niagara Falls, if you are on the Canadian side facing the falls, the sun would be setting either behind you to your right, or over the right side of the falls. There's a light show that happens just after dark, and the viewing area around the falls tends to completely fill up with people before that happens. I think that is a year-round event. That would make it hard to set up at ground level.
Sorry, I can't tell you about the towers since I have no experience there. My bet is no tripods allowed up the tower. The only hotel with a truly good view is looking from up river a bit, so it would not be so good. At river level, there's the boat ride, but once you get in close, better put your gear away because it'll get drenched.
If you are looking for something really different, you will want to go down into the gorge. Down river a ways is a park called Niagara Glen (I am pretty sure that is what it is called). There, you will find a stair case that descends about three stories, and from there you take trails down to the river. The trails go under boulders and it is very "primeval" feeling. That's a daytime thing, you don't really want to be down there after dark.
Even further down river is another place you can pay to take an elevator down, and walk along side the river. That's if you want to photograph a very powerful river with huge standing waves. (There is an admission fee for that.)
On the American side, you can get right beside the lip of Horseshoe Falls (so close it is scary), and you can also get down directly beside the American Falls.
Hope that helps a bit.
