I have both the Illuma-Lux and the Streamlight 3AA Luxeon. Physically, the Illuma-Lux is shorter, smaller in diameter and lighter than the Streamlight. In particular, the head of the Streamlight is a bit bigger, because it uses a reflector, while the Illuma-Lux uses an acrylic half-ball lens. The Streamlight feels very solid, like you could give someone a nasty whack on the head with it. The Illuma-Lux has a momentary tail switch, with twist for constant on. The Streamlight has a regular old click switch on the side of the body.
Beamwise, the Streamlight has a tighter hotspot, and throws farther than the Illuma-Lux, but still has pretty good sidespill. The Illuma-Lux is more of a flood, and has a large, round hotspot, good for lighting up a room. The Illuma-Lux has a noticeable yellow/green tinge to the light, especially when compared to the Streamlight, which is very white. I don't have a meter to take actual readings (I wish Quickbeam had an Illuma-Lux so he could do Lux and QUPS readings on it), but they both seem pretty bright to me.
These are my only LS lights so far, and I really like them both! I'm also looking forward to the Impact XL as another affordable LS light.
the beakman