from the discussion on "36 Reasons Streetcars Are Better Than Buses" at The Infrastructurist
Rockfish @1005a: "This is an interesting post in how it reinforces an implied “hierarchy” in transit where every tier feels compelled to trash the one below it... I LOVE trolleys as an alternative to light rail (which suffers from its own mania) but there’s really no need to trash buses to advocate for it. In fact, BRT is a realtively quick, cheap and easy way to establish rights-of-way and infrastructure that can be upgraded to trolley over time. Tey are not mutually exclusive."
But there are technological differences between the systems you mention. Trams/trolleys/streetcars operate in mixed traffic on city streets. Light rail operates on separate right of way from vehicular traffic, often grade separated. Light rail standards call for more infrastructure at stations to facilitate quick boarding, as opposed to a simple shelter or even just a sign at a tram stop.
Streetcars, LRT, heavy passenger rail, and HSR all accomplish different transportation tasks so you can't really say that one system is preferable to the one below it. The hierarchical ordering has more to do with the distance traveled, not the level of comfort. It would be impractical to build HSR to go from one side of a city to the other, just as it would not make sense to build a coast-to-coast streetcar line with a stop every half mile.
As for BRT, they say it is "cheaper than light rail" when really it should be compared to the costs of streetcars. BRT systems operate on city streets and narrow separate rights of way like streetcars, and their capacity is similar to streetcars. So it isn't appropriate to compare the costs of that kind of system with LRT systems that are fully grade separated, often in tunnels or up on viaducts and have complete stations rather than simple stops, and can move many hundreds of people in one vehicle.
BRT systems by design have a much lower capacity than rail systems, principally because buses cannot be coupled together like streetcars can. The frequency of BRT reaches a maximum level and then bottlenecks as one bus waits for the preceding one to clear a stop. The roadway of BRT degrades at a rapid pace and needs to be resurfaced every 2-3 years. BRT vehicles are cheaper than LRT vehicles, but have a much shorter service life and much higher maintenance costs, and are at the mercy of changing fuel prices. People always say BRT can be upgraded to rail, but really it cannot. The two systems don't share any common technology so there is no upgrading, only total replacement.
All that being said, there will always be a need for a variety of transit modes, including buses. Look at NYC, with one of the busiest and most developed metros in the world, they still depend on buses to keep people moving. The problem is that right now all our eggs are in one basket with buses, we need multi-modal transit and that means rail.