I believe the difference between running a single 18650 and 2 CR123s is voltage. I plan on trying CR123 (more voltage) in my shooting lights this year, I'm assuming the light would be brighter with them. Since it's only on for the shot length of run time isn't a concern.
I think cold has the same effect on either battery. If it's cold you're going to go through more batteries. The CR123 once done are history, the 18650 can be recharged.