Maybe OP from the UK is thinking about a scenic stop at either side of the Niagara Falls (and the casinos in support of local "economic development") - between Toronto & Buffalo's airport - as the buses, after the border crossing - would head south on I-190 and then I-290, passing Amherst (University of Buffalo a/k/a UB campus ... not much to see, really)

Downtown Buffalo is about 11 miles from the airport, typical 20-25 minutes drive time, except at rush hours. Greyhound and/or MegaBus would only stop at their designated terminal(s) and won't just let you off anywhere along its approved route, and a taxi ride just isn't worth it for the 2 hours in total as they are radio-dispatched (can't just hail them on the street as they don't cruise around, AFAIK ... locals) If OP decided to rent a car doing a one-way, then it's a different scenerio.

Concensus as others said, just enjoy the ride down & have a good bite, and hang out until boarding/departure time - it's a relatively small int'l terminal so getting thru should be easy enough.