As the name implies, the Stella 150 offers 150 lumens of light output (the Stella 120 offers - you guessed it - 120 lumens) and has an excellent beam pattern that concentrates the light directly in front of you. We've never done head to head comparisons ourselves but having used many lights on our bikes at different times we can attest first hand to the very efficient use of light on all Stella models.
Although Light & Motion categorizes their Stella models as multi-purpose / commuter models we think they're powerful enough to even be used in nighttime enduro race situations (editor's note: I plan to use mine in a 24 race this summer). At $300 this light is obviously more expensive than the basic commuter light, but it's a heck of a lot cheaper than a lot of race ready lights out there and we feel it's a great compromise between high end performance and lower end price.
The NiMH battery offers 2.5 hours of burn time in the high setting and it weighs a very respectable 328g for the light and battery.