Re: CMG Sonic-G vs. CMG Ultra-G What do you think
Basically, it's down to what you want: longer battery life or smaller size.
If you want a AAA light, then an Arc AAA would be much better than the Sonic (enough to justify the additional cost). A AAA light makes a great keychain light.
AA lights, like the Ultra-G have about double the battery life of AAA lights, but are also larger. Another good thing about AA lights is that they can use lithium AA batteries now (since they are available in stores now). Lithium AAA batteries won't be out for a while.
Personally, I would go with the Ultra-G. It is bulky, but I think it still works very well as a keychain light IMO. However, whether a AA light is too big for your keychain is up to you.
Of course, you could solve this problem by buying them both! (I'm pretty sure someone would suggest this sooner or later, so it might as well be me!)