I remember getting into this with my wife. She is into buses and those were black magic, I liked the subways. Took a few bus rides with her and she converted me. Both are great options with pros and cons.
Places where subways don't have as many stops, or cities that have better bus infrastructure and/or funding, likely (example: bus has more funding and as a result, you might only have to wait a couple minutes at a bus stop compared to 10+ at a subway).