Runtime's the problem. 18650s are far better than RCRs. 
+1
If you're using a 2xCR123 light that will take 18650, you're
much better off going that route. The capacity of RCR123 cells is really poor, and will typically only give you about 1/2 the runtime of a good 18650. If you must buy RCR123, AW brand is the best, IMO, but not cheap by any means.
Always buy protected cells!