@gemini My big reasons for more light rail rather than heavy is looking around at what's going on here in Europe (I'm in the south of france at the moment) where metro-level transport is all light rail/subway and heavy rail is inter-city. I don't know why since that's not my area of expertise, but I assume there's a very good reason.
Also I suspect heavy rail moves fewer people per dollar than light rail -- Montpellier moves a lot of people on their system and the cars are definitely cheaper.