Well, I'll take a shot at this. First, what they all have in common is they use a single LED (I think it's a 5mm LED) instead of an incandescent bulb. So while they will never burn out, are whiter, and have better battery life than an incandescent, they also will never be as bright or "throw" as far as an incandescent. 5 mm LEDs are best at providing more of a flood or wash of light for up close work, or lighting up a small room, not for spotting anything at a distance.
That being said, the main thing which distinguishes the Inova X1 from the others, is that it has a lens on the front that focuses all of the LED output into a narrow beam. While this means that it will "throw" farther than the CMG and Arc, it also in my mind negates the advantage/purpose of the 5 mm LEDs (i.e. to give a white flood of light). Projecting a dim spot of light doesn't seem useful to me, if you want throw in a keychain light go with a small incandescent like the Princeton Tec Blast.
The CMG Infinity and Infinity Ultra are basically the same light, the only difference is the brightness. When the Infinity came out, some people thought it was not bright enough, especially when compared to the Arc AAA. So CMG made the Infinity Ultra, which was much brighter then the Infinity, but with shorter battery life (25 hours as opposed to 41 for the regular Infinity). The Infinity Ultra is about the same brightness as an Arc AAA. Otherwise, the Infinity and Infinity Ultra are the same; durable, waterproof and runs on a common AA cell. If you want maximum brightness, go with the Ultra. If you want a dimmer but longer running light, or want a colored LED (like red for preserving night vision), go with the regular Infinity.
CMG Infinity & Infinity Ultra The Arc AAA is considered by most Flashoholics as the pinnacle of the keychain LED lights. It is about the same brightness as the Infinity Ultra, but since it runs on a AAA cell it is smaller and lighter than the Ultra, making it fit better on a keychain and easier to carry. However, this comes at the expense of battery life, with 5 hours advertised for the Arc AAA as opposed to 25 hours advertised for the Infinity Ultra (but take those runtimes with a grain of salt). Other than that, the Arc AAA is slightly more waterproof than the Ultra (50 ft as opposed to 10 ft depth), has tougher anodizing than the Ultra, and has a little better overall fit and finish. It's basically up to whether you want to pay more for a little better construction and smaller size for the Arc over the Ultra. I think the only difference between the Arc AAA and Arc AAA Premium is in the tint of the LED used (the Premium has a "warmer tint of white" <img src="images/graemlins/confused.gif" alt="" />).
Arc AAA Also, there are two other recent contenders in this category: the CMG Sonic, which is CMG's attempt at an AAA light similar to the Arc AAA, and the Arc AA, which is Arc's attempt at an AA light similar to the CMG Infinity Ultra. <img src="images/graemlins/crazy.gif" alt="" />
Basically, it boils down to the following differences. CMGs are good, durable lights at a reasonable price, while Arcs are better constructed, but at a more premium price. And, AAA lights are smaller, lighter and easier to pocket than AA lights, but AA lights will have longer runtime than AAA lights.
For reviews of these and many other flashlights, go to
flashlightreviews.com Hope this helps.
the beakman