The dome on an emitter transmits a different color depending on the angle from off center access. So If you look at an XM-L2 emitter straight on it will be a cooler and whiter color than if you look at it from on edge. There you tend to see a more greenish color. If you put one of the XM-L2 emitters in a reflector it will show that color shift as you white wall hunt with it. In normal use, it is not noticed very often.
The more dome-less emitters like the XP-L-Hi have this effect too but it is so slight as to not usually be seen, even if white wall hunting.
The XP-L-Hi emitter is by far the most requested emitter for my lights. But I offer any emitter you desire as long as it can handle the STFu power requirements.
The dome on an emitter will make the die look larger. Like you are looking at it under a magnifying glass, as in fact you are. That larger die image creates greats a larger hot spot. So it is true that the XP-L-Hi is a slightly better thrower in comparison too. This effect is slight but can be seen in side by side comparisons.
Finally, the XP-L-Hi is an amazingly powerful emitter and really handles high current well.
The XP-L-Hi is available in the following color temps: 6500K, 6000K, 5700K, 5000K, 4000K, 3000K, 2700K. The CRI of the lower color temps is higher. FYI, with Cree if a 4000K emitter is actually an 89 CRI it will be labled as an 80 CRI. So IMO it is important to visually compare the various emitters to see what emitter provides the look you desire.
Cheers
Dave