I had 2 Halcyon Scout xenons during my training, and 1 sl4 LED that stays in my drysuit pocket. The LED was VERY bright for the lights out exit in Ginnie, much more so than the scouts. HOWEVER, when we went to peacock and viz was 30ft at best, the xenon scout outperformed the LED because the LED couldn't penetrate as well with the particles in peacock at that time. If I had the choice right now, I'd have 1 backup LED, and 1 xenon/halogen, since both of them have their benefits. I like the durability and burn time of the LED, and the penetration of the xenon.
In a silt-out, I'm grabbing my scouts....in a clear system where my primary just happens to die, I'm grabbing the LED.
LED technology is growing fast, it seems every month or two a new LED backup or primary comes out that has us all drooling these days. I have a feeling I won't be carrying anything but LED's before long.