I will not be participating in the poll because the answer is application specific.
I'm going to lay out a few things to consider:
Most LED lights contain regulation circuits that can drag voltages below the ideal minimum recommended discharge voltages for either LiMn or LiCo cells. At least with a protected LiCo cell, you are only slightly over-discharging before the protection stops you from severely over-discharging the cell. With unprotected LiMn cells, you are prone to wind up severely over-discharging the cells, which is going to really kill capacity and cycle life if done repeatedly. On lights that run in direct drive towards the end of the discharge this is not an issue (buck-only regulated lights on 1 li-ion cell) as you will see major dimming before either is dramatically over-discharged.
The total stored energy of 2xCR123s, compared to 1x LiCo 18650, compared to 1x LiMn 18650 is hard to compare and requires first that you know what the load is. The 2xCR123 cells will beat the other two in SLOW drain applications, like 2+ hours. The LiCo 18650 starts to pull ahead of the CR123s when used in applications with ~1 hour runtime. The LiMn cell will beat both of the others in applications where you are pulling serious amps and just need a ton of power density. There are more issue to consider, especially the fact that most lights that are compatible with both an 18650 and 2xCR123 cells are not going to run in regulation on a single 18650, so the runtime on the 18650 is nearly double what it is on CR123s, but is diminishing output rather than regulated output.
Protected LiCo cells are very safe when used in proper applications, charged correctly, and disposed at the proper point in their life. LiMn chemistry is technically safer because if something does go wrong with it, it does not produce it's own oxygen during a burn. Both cell chemistries can burst, but the LiCo is more likely to produce additional fire and toxic fumes as a result. Both cell types usually come with PTC devices which will sever the electrical connection in the even of over-heating in an attempt to prevent thermal runaway. If the LiCo chemistry bothers you, then owning CR123s should bother you 10X as much.
My general recommendation is to stick with protected LiCo cells for any application with ~45 minutes or more runtime on the LiCo cell in question, especially regulated LED lights. Take advantage of the improved runtime of the LiCo cell. LiMn chemistry is best suited to making high power applications possible in smaller places, like a 1xRCR123 size light driving a P7 to ~2.8A, or a 2x18650 driving a bulb that draws over 3.5A.
but I don't care - to me it is not worth the risk when the reduced capacity of IMR and LifeP04 is acceptable and enough for all my uses so far.
