Actually, I'd say it really comes down to rails and tires. Streetcars can be given the same rights-of-way as buses and would perform similarly, except: the streetcar is a smoother ride. I can grab breakfast and eat it on a streetcar knowing there's only one direction of motion to worry about. On a bus, however, you can never tell which way you'll be jerked around. It really is much less pleasant, even if it's faster without the right of way. (Due to left turning cars)