Keep in mind that every LED is different from the next. Brightness, beam pattern, and tint. ARC uses the best LEDs and hand sorts them, but LEDs are inconsistent, and small variances are acceptable. Same goes for the LS.
The Premium will have a warmer tint LED but it may or may not be brighter, as hideo pointed out. Peter has stated this himself.
Quote from thread "ARC LE worth extra$10??":
"The Premium uses a warmer tint than the Standard. Both are the same in brightness (on average) and run time."
Quote from "ARC AAA Premium - First Impressions":
"For most people, the less expensive standard version is the best deal."
The beam shot above of the Limited Edition unit is noticeably brighter than the standard, but that is not always the case. With another comparison of another set, the results may be different.
The Premiums ARE QC'd at a higher level, and you get the PREMIUM designation engraved on it.
I don't want to dissuade you from getting a Premium, just don't want you to think the PE is better by a large margin.
I've been very happy with the build quality and brightness of my standards, though, so I decided personally that I don't need an LE/PE.