When it comes to transit, there is no 'mode'.

BRT makes a mockery of the 'best for' typology, largely because BRT is such a heterogenous category - quality varies from being heavy-rail competitive to being a bus with slightly nicer stops. (FTA labels aside, most 'BRT' isn't actually 'Rapid'--it lacks exclusive guideway).

American light rail hardly a 'clean' category either - tram-train is the norm, resulting in things like poor Portland (and formerly LA), where a through-corridor suffers a tram section in the middle. All are 'light rail' because they can't share track with 'heavy' rail - metros and commuter rail roads. Light rail is a vehicle crashworthiness standard. 

Having tried to identify all light rail stations (prior to the Transit Politic getting it done) actually transit is bafflingly heterogenous. Even with the 'New Starts' light rail systems, there are three distinct tropes: 1) tram-trains, 2) diesel, and 3) streetcars. 

1) German tram-trains (stadtbahn) are the trope definer for most American light rail - rapid transit service in the suburbs by using a former freight rail corridor, street-running in the central city (to avoid tunneling costs)*. In America, when we lack a freight rail corridor, we've built in freeway medians. 

2) New Jersey River Rail line is the trope definer, but Texas has built a lot of it. Save yourself the costs of electrification (and unsightly catenary) by keeping power generation onboard the vehicle. But that same fix makes stopping and starting both slower and more expensive, so stop spacing is much wider than for electrified traction**. 

3) Portland Streetcar is the trope definer for American streetcars. Portland's modern streetcar uses standard European trams, but that wasn't what got America excited about streetcars--rather, it was Portland's historic streetcars that were the original 'Development-oriented transit'. Supported by a local option tax along the corridor (American transit owes a huge debt to Powell's city of books). The whole concept of a 'pedestrian extender' and using a streetcar as a catalyst for real estate development in a proximate but inaccessible area (Pearl district). All of it street running from go. Over time, Portland has taken streetcar to the next tier, with a streetcar-only bridge, applying the same logic to places across a river. 

*If you put your city center light rail in a tunnel, so it's all rapid transit, it's evolved into a light metro.

** Battery technology may be a game-changer for diesel light rail, permitting both low-cost installation and electric traction levels of acceleration. Getting the brakes right is going to be the sticking point - batteries weigh a lot, so regenerate breaking practically necessary, and the 'trucks' (wheels) on rail vehicles are already complex and putting that much potentially explosive silicon near track is a bit perilous. 

Doubtful battery will be as relevant for trams/streetcars - anyplace that already has transmission lines will keep extending them, so the use-case is marginal--places without systems, for which the competition is BRT, which has a far lower initial investment cost and far better capability for incremental upgrades.