From a transit planning perspective, I can name one certain advantage rail has over bus--its resistance to loops and detours. The cost of an extra quarter mile of rail is a capital cost that must be addressed today. The cost of an extra mile of bus is an operations cost that is tomorrows problem.
Further, once determined, a rail alignment is SET. It is very difficult to add a loop or detour at a later time. I was recently enraged to discover that UTA had added a detour to the 205 route between my place of work and the FrontRunner commuter rail station. The detour is only one block, but requires two left turns, through congested signalized intersections, destroying the routes reliability, and thereby it's value as a connection to FrontRunner.
I become increasingly enamored with busways--dedicated fixed guide-way at a fraction of the cost of rail.