There are size differences.
2x123 lights
E2E/E2O uses a MN02 or MN03 bulb with different brightness. It is very small, very compact and puts out a very respectable amount of light.
6P is another light that allows the use of a P60 lamp assembly. It's slightly brighter and the beam pattern is slightly different from a E2 series light. It can throw slightly furthur
C2 is a modification of 6P with a nice grip ring so that you can hold it combat style, along with a piston, with your index and your middle finger gripping the ring and your thumb pressing the tail cap switch, very comfortable if you are also using a pistol (as these are tactical lights, they designed to be used in tactical situations)
M2 is a C2 with a modification to the bezel. It has shock isolation which will help lengthen the life of the lamp assembly because when you fire a piston, there is tremendous force backwards.
G2 is made using nitrolon, lighter than aluminium and much more scratch resistant as it won't show up unlike on aluminium
G2Z is a G2 with a rubber ring that allows you to grip it combat style, like the C2.
G2Z has a nicer grip, is lighter, has pyrex lens which allows higher light transmission, doesn't melt under the high temperature that higher output Lamp Assemblies will produce. It's a good starting Surefire light.
2x123 lights
E2E/E2O uses a MN02 or MN03 bulb with different brightness. It is very small, very compact and puts out a very respectable amount of light.
6P is another light that allows the use of a P60 lamp assembly. It's slightly brighter and the beam pattern is slightly different from a E2 series light. It can throw slightly furthur
C2 is a modification of 6P with a nice grip ring so that you can hold it combat style, along with a piston, with your index and your middle finger gripping the ring and your thumb pressing the tail cap switch, very comfortable if you are also using a pistol (as these are tactical lights, they designed to be used in tactical situations)
M2 is a C2 with a modification to the bezel. It has shock isolation which will help lengthen the life of the lamp assembly because when you fire a piston, there is tremendous force backwards.
G2 is made using nitrolon, lighter than aluminium and much more scratch resistant as it won't show up unlike on aluminium
G2Z is a G2 with a rubber ring that allows you to grip it combat style, like the C2.
G2Z has a nicer grip, is lighter, has pyrex lens which allows higher light transmission, doesn't melt under the high temperature that higher output Lamp Assemblies will produce. It's a good starting Surefire light.