These Li batteries that we/I use produce a voltage of 3.6 to 4.2 volts depending on charge state. Generally 18650 relates to the physical size of the battery ie 18mm diameter by 65.0mm long. Energy density or mA corresponds somewhat to the size of the battery.
CR123A = 16340 is around 800mA
18650 is around 2200mA
I use both these but ther are several other sizes and capacities available. The important thing to remember is to use cells with protection circuits built in. This protection prevents over charge and discharge of the cell. There are some dangers involved in using unprotected cells, google Lithium-ion battery dangers.
I haven't conciously chosen one cell over the other. I use a pair of
small torches mounted to my helmet and was using the CR123A batteries till got sick of stopping to change the batteries after about an hour of ridiing. I had a few 18650 torches floating around the house and work and decided to try and build an external battery pac to get better runtimes seeing as the voltage requirements matched. I was toying with the 18650 batteries while sucking on the mints and I looked at the Eclipse tin, the battery, the tin well the rest is history.
Now I ride from 7pm to 10pm with a total of five led lights unless you count the 1 SSC P7 as another 4 without any dimming or stopping to change batteries.
Les