I don't believe there's a discount for pre-booking, you just ask at your hotel and they call for the bus to stop at your hotel to pick you up. All the major hotels have the brochures in the lobby and you would certainly never have a problem getting a place if you book when you arrive.
Hi, we just got back (Thursday 21st April ) from 5 day trip in Toronto . Checking the cost of the coach day trips to Niagara Falls , from Toronto , they seemd not right fit ( 120 dollars ? we could get own lunch and not pay 20 dollars each, we were not too bothered with Niagara on the Lake , etc) . So we took VIA train at 09.40 from Union Station.(This train goes to US New York and back once a day ) Drops you at Nigara Falls train station 2 hours later. From there , across road is main bus stops and 4 bus routes take you to drop off point town centre/near falls (10 minute bus ride, bus fare usually 2 dollars but special offer at 25 cents !) . Alternatively the walk ( back) was something like 35 minutes from Falls to train station for return train at 17.45. ( Train was late departing because Canadian Immigration were checking passengers entering Canada but made up the time en route) . Some special offer with VIA meant we paid 32 Canadian dollars each instead of the usual 64 dollars each. We had several hours to wander around Niagara Falls town and the Falls themeselves on the Canadian side. For us , this trip worked out perfectly because we wanted to see the Falls and not really the town ( think neon lights / slightly seedy / fast food places / a million hotels all claiming to overlook the Falls!) We also were not bothered about crossing to US side by walking across Rainbow Bridge . Hope this helps!