Basic Photon configurations:

Photon 1 - squeeze (momentary) only, simple and reasonably water resistant. Change batteries with pocket knife or strong fingernails.

Photon 2 - squeeze (momentary) or mechanical slide switch (constant on), simple, water resistance compromised because of the hole where the slide switch is. Need #00 Philips screwdriver for battery change.

Photon 3 - computerized, two button interface, 3 levels, I don't like this one very much. Not sure of battery change even though I have a few of them.

Photon Freedom - computerized, 1-button interface, dimmable, flasher modes, reasonably water resistant, change batteries with pocket knife or strong fingernails. Drawback: can turn on in your pocket and kill its batteries except in signalling mode which adds complexity.

Photon II is my favorite of these, not much can go wrong. If it gets wet, open it dry it out. Battery change isn't that big an issue, you do it at home. In the field, carry a spare light.

LED color: red, orange, and yellow LED's run at low voltage and use a single CR2032 cell. These aren't super-bright but have TREMENDOUS runtime, like several days nonstop. Green, blue, and white (i.e. shorter wavelengths) use higher voltage (two CR2016 cells, lower capacity and half the thickness of the CR2032) and are much brighter but don't run nearly as long. Green was originally shipped with a CR2032 and brightness is not that bad. White and blue can run on a CR2032 but they are rather dim that way. The Photon Freedom's dimmability makes this unnecessary--on low mode you get lots of runtime.

I have a white Photon II and a red knockoff on my keyring. They don't take much space and the red is handy for night uses and can provide days of minimal light if needed.

See the links I pasted earlier for cheap Photon II clones.