Whether it be on a Traveling Professor tour to Paris or traveling alone to Italy, I always prefer taking the bus over the metro. Granted, the metro is probably faster, but who wants to travel in a hole in the ground?
This is why I like taking the bus:
Less stairs: if you have been to Paris or Rome, you know that taking the metro involves climbing up and down the stairs. For all intents and purposes, that is eliminated by taking the bus.
Better scenery: As I mentioned, who wants to travel in a hole in the ground? Traveling by bus gives the visitor a much better perspective of how the city is laid out.
Safety: Let's face it, you are more likely to encounter a pick-pocket or unruly crowds down in a metro station. In case of an emergency, it is probably easier to get off a bus than out of a metro station.
More sociable: For some reason, people seem to be more friendly on the bus.
More laid back: Bus drivers will provide directions, they wait for you to board (unlike those closing doors on the metro), and I find it to be a more pleasant experience.
Better coverage: In Rome especially, the bus routes are much more extensive than the metro routes.