And/or runtime, as the OP mentioned. I carry an E05 for the output and beam quality for routine use in various situations and I don't worry about how long the battery lasts. However, if I were mostly concerned about having an emergency light source on my keychain for when I'm unexpectedly plunged into darkness, I think I'd lean towards the E01.

Whenever there's some power outage for whatever reason, I always shake my head whenever I read about people trying to use the screens of their mobile phones as a sort of makeshift flashlight as they stumble around in the dark. It's so easy to have a decent flashlight on you always. Phones make poor flashlights, and that's an incredible waste of battery life for a valuable tool.