Light rail is just about the most energy efficient form of motorised transport and it is likely to be more successful in getting car drivers to switch modes. However, it should not be a matter of rail vs bus, though I acknowledge that there is only so much money being made available.
FOSBR are right to oppose BRT, not least because of unsuitable routes and new roads, but I'm not so sure about all their reasoning. The case I'd make against bendy buses is that they have potential problems with: insufficient effective motive power; slower speed and acceleration due to the extra weight; overheating leading to stalling, or even a fire if diesel fuelled; in crowded areas with narrow streets and tight turns the accident rate may exceed than conventional buses. Bendy buses are supposed to be highly fuel efficient but this must be dependent on the city and the system they are running on and so in practice I have doubts that they will be more efficient in operation than double deckers here in Bristol. Mind you some parts of FOSBRs rail alternative are non-starters - a tunnel under St Mary Redcliffe is both impractical and likely to be far too costly.
More thoughts on sustainable transport here: