Trams definitely carry more people than buses, even articulated ones. On top of that they produce no local pollution, as they run on power that is not produced locally (and which might actually come from clean sources of energy). As per for the "trams are slow" argument that doesn't really count. Most of the time cars are riding bumper to bumper during rush hour, while a tram has its own dedicated line which makes it get ahead of cars almost all the time.

Source: me, who's riding the tram to work and back most of the times.