How can you decide? Well, it's worth looking at if a city has an average density of residents and jobs of 30 per hectare or greater, because it is most likely to benefit from investment in public transport infrastructure ("Order Without Design", Alain Bertaud, 2002). Examples of cities with metros are London with 32 per Ha, Paris with 88 and Hong Kong with a staggering 367. Just to disprove the rule, Atlanta, GA, USA has only 6 but it has a metro system.
If you are looking at suitable station spacing, for a metro it will be around 1000-1600m in a city centre. Stops on an LRT system will be around 600-800m apart.
Broad budget project costs: >USD15million per km for a street level LRT system; >USD70million/km for an elevated system; >USD200million/km for an underground system. These "ball park" figures exclude land acquisition costs and will vary widely by country.